Welcome to the forum Simon, lots of helpful builders on here
As for sourcing a Mig/hiring one.....
If poss try to get your own Mig, the reason being....you,ll be surprised how useful a tool it becomes, its not just about welding your chassis together, it,ll be there for when you need to "just add a bracket" here & there later in the build, nothing more annoying than not having a Mig available to do that "5 minute" job
Plus once you own your own mig, you,ll get "a feel for it", how it responds etc & end up using it with confidence like using your favourite spanner etc
I bought my mig from a for sale thread on the Mig welders forum , its a 151te clarke gas mig, even came with 2 pub style gas bottles too, all for £50.
Make sure you get a good auto darkening helmet, really does make the world of difference when welding.
Thanks to all with regards my chassis flat packs
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Whichever route you take Simon, I wish you all the best for your build.
cheers
andy