My new cage, waiting for its occupants

My rats were fed on a similar diet to what this breeder is recommending, you can pick rabbit food up cheaply enough from a local feed store (usually just a lockup on an industrial estate). Alpha herbal used to be one of the most recommended but they changed the recipe and it got very dusty and my rats stopped eating it. Another is Harrison's banana brunch.

If I were you I would modify the cage door, use pliers to remover the hinges and turn the door upside down, you can add thin chain or string so it only opens to 90 degrees and acts like a drawbridge, much easier for getting ratties in and out.

One thing though it is illegal for the breeder to let you take the rats at 5-6 weeks, 8 weeks is the youngest an animal can be sold in the UK, I would highly doubt any reputable breeder would let them go so young and most prefer to keep them to 10+ weeks. Is it a registered breeder? Have they earned a stud name? How many litters do they have a year? How often do they breed each doe?

It's looking good I'd fill it up with a few more levels to make use of the horizontal space, and I agree with the poster about flipping the door (saying that, are you sure the panel has gone in the right way up? Maybe they've redesigned the cages since I had rats but I'm sure Ferplast used to put the doors at the bottom, opening downwards).

I would either buy some spare corner shelves (I think PAH can order them in, or try eBay perhaps) or make your own levels. Large cat litter trays may fit, suspended from the bars, and if you get a deep one you can fill with substrate for a good digging box (my boys were never fed from a bowl, I scatter-fed in a litter tray on the top level so they had to climb up and forage for food, kept their minds busy and their bellies a bit more trim!). Several more hammocks (rats can never have too many hammocks!), some tubes for them to scale, rope toys, etc. - young rats will be pinging all over the place, so give them plenty of objects to climb on, but ensure they can't fall a great distance if they misjudge a jump or loose their footing.
http://www.fancyratsforum.co.uk/ was a good forum when I was on its predecessor, I imagine it's probably still a good place for advice, cage inspiration, etc. nowadays.

www.ratrations.com is a good one for food as well as accessories - they sell various mixes, to save buying in a large amount of rabbit food and mixing your own (just bear in mind that not all rabbit food will be suitable - some too high in indigestible alfalfa that won't be eaten, some coated in syrup to make it more edible, etc.). They also stock several suitable substrates (what you put on the base of the cage) and litters (for the litter tray) - shavings should be avoided, as should other dusty substrates like wooden pellets, hay, etc.

I understand you not naming on here, I may sound over cautious but while I did have pet and show rats from breeders as my own pets I also ran a rat rescue for just over five years and most of my residents were from pet shop lines or irresponsible breeders so I always tend to assume the worst when it comes to rats (and a lot of other animals if I'm honest). If it's the breeder I think it is I don't know her personally but have heard of her and know the breeders she started her lines from and I trust their judgement that she is also reputable.

As for the litter tray, yes they will use it, I've found does tend to use them more than bucks.
Some rats use them with no training at all but as long as there is different litter to the rest of the cage it doesn't take long to train. Start by working out which corner they prefer to use (they all tend to prefer corners, just hope they all choose the same one) put the litter tray in chosen corner and several times a day scoop any mess from the rest of the cage and put it in the tray. While they are learning even when you clean it out, put a small amount of "dirty" litter back in so they can smell their toilet area. It shouldn't be more than a week or two until they use it all the time.