Welcome aboard Tom!
I can't help it, I'm already both charmed by your post description and tickled for you that you're getting back into rats.
You paid only 40?? Oh my, you got a deal that most rat owners would sell their kidney for. That is a serious rat mansion from the dimensions and could easily house up to 12 or so rats if they all get along well and you set up their space well. Beware, they're like potato crisps in that you can't stop at one. Colonies tend to grow to fit the space! "Gotta Get More Rats" is a real scientifically quantified disorder.
My first question is, what is the bar spacing? In the US, it would need to be 1/2 inch to be suitable for babies and females as they can squeeze their heads thru tiny places. 1 inch spacing is suitable for only the biggest males. This being a professional cage, I'm assuming it'll be close to 1/2 inch, but always good to double check.
Bless you all to pieces for doing your research /before/ bringing your rats home. So many people don't do that. I'm a compulsive researcher so I too was a research before I commit person. I didn't appreciate how rare that was until I was active in the hobby for a while and saw how many people just won't do it.
I started out with a colony of six males, and now I have a mixed colony of five females and three males-long story for later. I've only had female for about three months and males for 3 years.
In my honest opinion? Scent/marking wise, the difference is academic. They both pee to high heaven. Males are a wee bit stronger smelling, but not as much as you'd think. Females smell somewhat like grape soda. Males smell like warm tortilla chips. Males will secrete an orange stain on their shoulders called buck grease. It can be a more intense tortilla chip smell and be quite musky. Minimal bathing, good diet, and feeding them a drop of olive oil weekly and it'll cut way down on that.
If you have two rats in a cage built for 12, as long as you clean the cage once a week, you won't have much of a scent issue.
Rats don't smell nearly as pungent as other rodents.
The bigger differences between males and females is personality. Females are usually busier, sillier, and tend to chew more because they like nest building. Girls are less likely to want to hang out in your lap or like to be held. They gotta go go go! They are a bit more receptive to trick training because of it.
Males, as they age, turn into lumbering, lazy squishes. They're couch potatoes.
Health wise, females are much more prone to re-occuring mammary tumors that have to be surgically removed. My girls are mostly older girls, and in my 3 months, I've removed 3 mammary tumors from two girls. Spaying them while they are young will drop the occurrence of mammary tumors /way/ down. In the US, the price of a spay can range from $90 to $300 depending on how badly the vet wants to gouge the customer, and it is a very invasive surgery with long recovery time. The cost of a mammary tumor removal is $60-200 and the risk of repeated surgeries.
If you've got a reputable breeder with a good line that has low or no occurrence of tumors, then not spaying might be a good gamble.
Males, if poorly bred, can be prone to hormonal aggression that pops up around 6 months and peaks at about a year. At the first signs of it, if you get them neutered, that will usually nip that in the bud. Neuters run about $30-150, and are much less invasive with minimal recovery time. Plus, two weeks after a neuter, they can be housed safely with girls.
Males are also more prone to gradual paralysis of their hind legs as they age, a condition called hind end degeneration or HED.
As for tips....wow. That's a wide open request. LOL
You've got the right cage, you're already doing the research to find the right rats /and/ willing to wait to get them.
My first tip, would be to research all veterinarians within driving distance to you. Find out who specializes in rats, or has had a lot of experience with them. Price check them and reputation check them.
I started by cold calling vets in my area, and even if they had no rat experience, I'd ask them who they'd recommend. If they say yes, ask them what kind of price range they'd charge to spay or neuter a rat and see what they say. They are often reluctant to quote price over the phone, but just stress that you're not looking for exacts, you're looking at a typical price range just to research and get a feel for what's typical.
The biggest drawback for rats is that ounce for ounce, they're unfortunately one of the less healthy critters you can have. This is because of their long history in the medical testing world, and the ease and speed of unscrupulous breeding. So a good, reasonably priced vet is worth his or her weight in gold.
Research what common rat ailments are common in your area. Being in the United Kingdom, you have a lot of good breeders within a day's driving distance. The US being as spaced out as it is, makes finding good ones much more of a problem.
Become active on UK pet forums. It won't take long for you to figure out who really knows what they are talking about locally and can give you some helpful insight unique to the veterinarians and rats where you are.
In the meantime, it's awesome you're here. We'll be happy to offer you specific tips as you go along, but there's so much it might be easier to ask specific question all along.
Oh, what I can do is this...
With all of my personal research, I created several blog pages designed specifically to answer common new rat owner questions (the questions I asked myself).
Here's the direct links to them, though you can also get to them by clicking on my name and going to my profile page:
Where to Get Your Rat: http://www.paw-talk.net/forums/blog.php?b=239&goto=prev
Selecting a Rat for You: http://www.paw-talk.net/forums/blog....3936&goto=prev
Expected Cost of Keeping A Rat: http://www.paw-talk.net/forums/blog.php?b=3936
Bringing Home Baby: http://www.paw-talk.net/forums/blog.php?b=3941
Free Range and Rat Proofing Your House: http://www.paw-talk.net/forums/blog....3936&goto=next
(based on what's available and what to avoid in the US-some of which will apply to you): http://www.pxrats.com/ratfood.html
Posted diets in the UK:
Brandywine Rats Diet Page: http://www.rattycorner.com/
(you may need to click on 'care' and then 'feeding)
Milliways Rat Diet: http://milliwaysrattery.co.uk/ratcare.html
The Shunamite Diet: http://www.shunamiterats.co.uk/shunamite.shtml