That Marin should have some decent parts but no denying that some might need replacing but a general £20-30 bicycle tool kit should suffice if you don't have one already. This sort of thing but it would equally be needed for your Indi bike which will have lower spec components too and could be more problematic. Anyway this sort of thing. Also that Marin was just an example close to me it will be different bikes close to Max1980. Looks a simple bike to me to sort out although might need a bit of brute force in a few places.That bike looks like a total money pit to me. Plus you would need time, workspace, tools, skills and money for parts. I mean, it would be fun doing it but I have no idea if OP has all that.
£600 is nearly an impossible budget but you could get a Woosh kit for £540 and one of these very simple, brand new, steel, Indi 'mountain' bike for £140 from Halfords for a total of £680.
But you are still going to need a few tools and know how to work on bikes.