This is a bit off topic, perhaps a new thread would be better.
I wouldn't mess about with the small disposable bottles if I were you - they don't last and I'm sure they leak when stored. Which means whenever you go to do a job you're always out of gas.
It does make it easier for having different gasses though, and while full size bottle rental is not cheap but you do get a much better choice of gases. If you do it right you can swap and change gas types and pay only one bottle rental, or have a gas for a one-off job without paying rental if you get the bottle back over a weekend for example. The MIG plants for small bottles and large bottles are essentially the same, but just have a different regulator - I think a regulator for the disposable bottles is about a tenner, so I have both. The large bottle MIG plants usually only take 15kg reels of wire. A 15kg reel of 308 (stainless) wire is about £100. Chopping and changing between mild, stainless and ally isn't that easy as you might think as you have to have different gases and you have to do a full clean down/change the liner etc between different materials otherwise you can get contaminated welds (esp. switching to ally from mild/stainless).
I don't know anything about the gasless MIGs, apart from the wire is expensive and limited in choice and that they're not as good as normal MIG as you don't get the control. Fine for welding gates etc
I always wish I'd have gone for an inverter kit, but getting all the bits and bobs together is very expensive.
Surprisingly large amount of welding stuff on eBay - I got a deal on a new Clarke 160 Turbo on there last year. But if you're a bit of a noob I would get friendly with a local Engineering and Welding supplies place and perhaps pay a bit over the odds for the free advice you'll get. If they're flogging you a welder, make sure you can get spare parts by coosing a branded one.
You'll want screens, grinders, grinding discs etc etc, all from your local welding place...