Hiya Dan,
Good news on the nitrox course.
Gear - Depends on whether you want to dive independent or manifolded twins.
If you're going for independents, then a tekwing with Buddy bands (i.e. NOT steel ones) is a good option. Then you don't need a backplate (unless you want to switch to a different one later). This rig can also be used with a single for holiday diving etc. Should come in around £250 e.g.
Kent Diving - Buddy - NEW Buddy TekWing - BCD's
There are other ways of doing it of course, but this is a cheap option, and it would leave you £150 odd for an analyser which is about the right price. IIRC you already have all the regs etc that you'd need. You might want to think about a third 12l cylinder as well, so you can swap in a spare before the second dive etc.
If you want to dive a manifolded set, then to be honest you'd be better off buying a complete twinset rather than converting your singles IMHO. You are also talking about quite a bit more £££, and might want to be looking at a different wing setup as well in my view.
The decision about independents / manifolded should be made on the basis of how you want to dive, not what you can afford - hopefully you're already clear on that.
Cheers,
Tom