1500 isa VERY good budget.
I bought an Oxford mig (exactky the same as tec arc and portamig, g=different colour/label) and it was night and day compared to clarke, which in turn was way better than SIP.
A local chap who is a retired bodywork man and lots of high end experience has just bought an R tech mig, and he is absolutely delighted with it. Nice and light too being an inverter welder.
Saying all that, if just doing car bodywork, I was really impressed by a snap on mig 130. Bit ancient, but really nice quality.I have seen the occasional NOS one up for sale.
Gas bottles are no problem, I get mine from a local supplier, and there is Hobbyweld, Adams and others.
Best bet is to visit the Mig Welding forum.
https://www.mig-welding.co.uk/forum/
As well as a mig, you need at least one, better 2, angle grinders. A decent self darkening helmet. A selection of clamps, and a joddler/hole punch (don't skimp here, cheap ones are naff, Tama are excellent, had mine 30 years.)