Sorry I'm late 
Great to see you've been reading up on them a bit :) There's a lot to learn but equally, once you know it's easy to remember :)
I'd say 3. We have a pair of boys and have 3 girls. We actually had a pair of each but when one of our girls got a tumour we had to get two more as we couldn't leave her sister as a lone rat. For that reason I think three is good. Younger rats are less likely to get tumours or die, but they can have some pretty scary accidents and all operations with rats hold a (small) chance of losing them, so if that happens you don't want to be scrabbling around trying to find a new rat or two whilst you and your existing rat are grieving the one you've lost. One of our boys had to have his eye out at 3 months old and we did panic a bit and start contacting breeders just in case we needed to get a new rat ASAP.
Also, we found that their behaviours changed between 2 and 4 (and then 3), it was interesting to watch them interact in groups. I do wonder what it'd be like to add two more to our boys cage but our landlady has vetoed any more unless we're in the same situation as before (one of the boys dying and needing to get him company).
RE the conservatory, I'd avoid that. They do need a pretty constant temperature. From July to last week our rats have been somewhat sluggish in the middle of the day and we live in a cooler basement flat. Today is the first time I've seen them running around during the day in a while actually. But the conservatory would probably be a great place to free range them as long as it wasn't too hot/cold.
I use a variation of the shunamite. It takes tweaking and you learn from your rats what they will and won't eat. Mine won't eat the smaller grains, for example, but love those barley ring things, so I've tweaked it and order in more of those now. I did put in a lot of seeds in my seed/herb/veg quota, but they don't like them so much but they love the dried veg and herbs, so more of those will go in from now on.
www.fancyratsforum.co.uk/viewtopic.php?f=14&t=56
I use the above version, I don't know if you would have found it, I stumbled across it when looking for ways to improve my mix a few months ago. It is the updated Shunamite, but the actual website was never updated. I get most of my ingredients from Rat Rations, it's cheap, extremely quick delivery and they've always been very good at handling my queries/complaints/suggestions about products. They have pretty much everything you need to make up your own muesli.
The problem with muesli is that you may have picky eaters. Ours love the dried fish we have this time round, will happily leave the grains. We're going to start mixing in a bit more pellet-type food into our muesli (they love Supreme Science Selective Rat Food) to even it out a bit.
We don't have a set recipe as we just make ours up from whatever we have in roughly the right ratios, mix in the local breeders bags occasionally for variety, chuck in different protein sources, etc. We have a massive box that we mix it up in and keep a smaller box and scoop by the cages.
We don't have family nearby either, managed to find a lovely woman who'd owned rats for 20 years before her partner vetoed them for a couple of years, so they compromised that she was allowed to rat sit for people instead!
Where in the country are you? I can wholeheartedly recommend an NFRS breeder and a rat sitter near Brighton, and also recommend the NFRS breeder we got our boys from in Kent :)
I'm also going to say that this cage is brilliant:
www.littlepetwarehouse.co.uk/hamberley-single-metal-large-cage-p-12607.html
We didn't want all metal but our rescued girls were chewers so we had to make the leap, but it's such a good cage that I don't regret it! We're actually going to replace our boys cage with another of these when we can afford it.
That site is also good for rat bits, but do not trust what they say is "rat proof" or "suitable for rats", a lot of what they sell as suitable is not. The wheels being the primary example. Once you get to know rats you'll get to know what is/isn't suitable, I still check things on rattit or on the facebook group I'm on if I'm not sure.
I don't know if you're planning to get a wheel (they don't get much use out of them tbh), but as this is something I'm anal about I'll say it: please make sure you get one that has a solid running part and no extra wires attached to it. Anything with bars or mesh be wary of as they can get feet caught whilst running and break a leg. Anything with bars going across the diameter they can get a tail lopped off by. Rats need big wheels. 12" or so. Small wheels can damage their backs. Giant Silent Spinners are probably some of the best wheels for rats, though you can get cheaper ones. I love the look of the flying saucer style wheels too, apparently rats like them too but I don't know anybody with one.
So this:
www.littlepetwarehouse.co.uk/blue-metal-large-28cm-degu-chinchilla-wheel-p-16673.html
Is a no go. The bars across the wheel and the frame can decapitate them even though the wheel is actually solid and it's a good size.
Girls are more prone to tumours but you can reduce that risk significantly by spaying them young. Our babies were done the week before last at 11/12wo. I don't see a huge difference energy-wise between the boys and girls though. That's more down to the personality of the rat I've found. One of our babies will sit in my bra for an hour given the chance, the other is a shoulder ratty, one of the boys is very much a shoulder rat but I can get the rare cuddle out of him when he's sleepy and docile. Our older girl gives us cuddles when she's in the mood too. All of them run around like mad when we're free ranging. The boys are straight behind the computers and the dark spaces on the edges of the room, the girls only really free range on the desk. We have just started letting them on the floor and it still freaks them out at the moment!