rofl yeah i seem to remember MMA welding a carmany moons ago. 1.6mm rod and large brass blovk to sink the heat (less holes).
I don't really do a lot of welding these days but i am a coded stick and mig welders as well as extensive experience with TIG and gas welding. including welding of Bronze, copper, high carbon steels, aluminium and stainless.
heres the thing with welding, the best and easiest weld is accomplished through cleaning the area to be welded correctly and back to metal, if you have lots of rust chase it out and replace with good plate. Forget completely any form of gasless mig welding (waste of time in all honesty). use argon/co2 mix for mild steel i.e car bodies.
to setup your welder you will require a spare piece of same size plate and practice on it to get your current correct. you don't want to be burning holes everywhere, garages rarely offer a proper welding service basically most of them can satisfy an mot inspection but truth be told this full welding balls came along because the is expected with so much welding it will be properly done in a few places lol, sills at the factory are spot welded.
good car body welding is not at all easy, especially for a novice. thats becuase it is positional, meaning you cannot move the work so you have to be able to weld at almost any angle to an acceptable standard. if you do attempt this job remember flashes from welders can cause anything from sore eyes to temporary blindness!! so select a good shield and probably a number 9 dark glass for such low current welding.
i suppose this leads really to you should if at all possible get an experienced expert welder to look at it for you, it is rarely neccessary to completely replace a sill on any car, just to get it through an MOT test often patching the old one to get a pass is very much more efficient. the chassis welding around the wishbone also would likely not in itself be a big problem chassis even on a KA will not be too difficult to put right with welding.
the charging out rate for a qaulified welder is likely to be in the region of £40 per hour so £300 does seem a little on the high side. but then again if that is for a full sill weld then the car would have to be stripped out inside because of the fire risk!!! this is very high on that sort of repair so make sure if you do it yourself you have someone watching inside even after carpets pulled back with a bugget of water on standby.
well i probably put you off but with experience it really isn't impossible to do it your self at all, good luck with you efforts.
tbc