For most people the decision to build a Kit
Car will mean a welder will have to be purchased if they plan to make the
chassis rather than buying one.
The cheapest type of welder available is an arc welder at as little as £50 new. Arc (or stick) welders use flux coated welding rods, the flux aids the welding process by both providing a “smoke” shield” for the weld preventing oxidation and by floating to the surface of the weld mixing with various impurities and forming a coating of slag. This layer of slag has to be chipped/cleaned off the weld once it has cooled.
Apart from the disadvantage of having to clean up the welds, arc welding is considered more difficult than MIG welding especially on thinner material.
Most builders go for MIG (metal Inert gas) welding where while the weld is being laid down it is protected by a shield of inert gas, preventing an oxidation reaction with the air.
Welders are usually describe by the maximum current they produce. Hobby welders can be had from around £120. They will all have a stated range for steel thickness. Most of the small ones (90 AMP) will go to 4mm. As with all these things, if you try pushing it to the limits it will become a struggle. For most cars the welding is 16 swg or 1.6mm. There is a bit at 3mm (suspension brackets etc) and the seatbelt mounting plates need to be fairly beefy. A machine capable of 5mm should see you through.
The other aspect is duty cycle. The more expensive machines, around the £200 mark are fan cooled and so can be used continuously for longer time. Unless you are going to try and weld a whole chassis in a morning, duty cycle is probably not really an issue.
Most of the car is short runs and tack welds of an inch or so lengths. In short a welder at around £150 - £170 from Machine Mart or similar will be OK for a kit car build without needing to push it to the its limits.
Slightly cheaper than MIG welders are so called gasless MIGs. These are similar to MIG welders but do not have a gas bottle. The shield for the weld comes from using a welding wire which has a core containing a flux. Cored welding wire can be up to 4 times the cost of ordinary MIG wire so although the welding machine can be initially cheaper the cost of consumables can soon erode this advantage.