A cheap arcwelder , capable of 120 to 140 amps, costs in budget friendly shops around £65. And comes most of the time with a small pack of 2.5mm electrodes, a cheap welders visor and a cheap chiphammer and wirebrush.

With this you can weld most mild steel applications in the robots.
Stainless or hardox is not out of the question , even with good results. But it will take a lot of practice.

It's also the cheapest welding you can do. A MIG or TIG needs argon and specialised welding wire.
A gasless mig, that needs the expensive filled wire. Oxy-Acy is very consuming in gasses, and those ain't cheap either.

One word of advice, when welding, make sure the clothing you wear is pure cotton, wool, or treaded for the purpose. You don't want flaming and/or melting nylon, polyprop or simular clothing..