The API rack and lunchbox are very competitive when you look into all the options, and they're readily available here in the UK.
If you're buying a lunchbox used you want to make sure it's one of the newer HC variants which has a higher current supplied to the modules. The old ones run API stuff fine, but some of the newer modules that are floating around demand a lot of juice.
The Lunchbox suffers from an issue whereby the PSU is oriented in such a way that the module closest to it can sometimes pick up noise depending on what that module is. The 10 slot rack doesn't have this problem with its external PSU, but the external PSU is obviously slightly less convenient. I also get a bit nervous about mine because the cables are fitted to the PSU with glands rather than being separate cables that I can get a spares of. It also makes the PSU unit harder to pack neatly into a flight case. Overall I think the 10 slot is a good deal.
Bargains can be had, and API stuff is plentiful here. Shop hard.
Be aware that despite there being a million and one new 'amazing' 500 series modules, the format is best suited to mic pres. It's a small space and the front panels get very fiddly very quickly, especially if you need to recall stuff. That said, there are some very good eq's and compressors out there. Some are 2 slots wide in order to solve this problem.
If you're DIY handy, the ClassicAPI mic pre kits are extremely good. I have a pair of VP26's with VP25's on the way. I'm planning a whole rack of them. They have the quality that you want from API (tight low end, present midrange colour) but without the tizzy brightness that can make a modern 512 a bit brash. They extremely economical. The downside is that you have to build them, and there's no come back if you get it wrong. But I'm genuinely not good at all that stuff and I found it took three hours for each VP26, they work perfectly, sound better than anything else I have on guitars and drums, and they have an output attenuator, which is something you need to think about if you're buying 512's. You could save hundreds on these and get better pres. In my opinion of course.