I started with a garage with brick walls half way up then asbestos sides and a leaky flat tin roof.

First replaced the roof - it went

EPDM membrane
Ply deck (can't remember thickness - 1/2 or 3/4 inch?)
50mm gap
100mm foam insulation (the white stuff) between the rafters
Vapour barrier plastic sheet
Ply lining (ISTR 10mm)

Sides - didn't touch the asbestos, just built inside it. Walls were uneven so added studding to make the walls vaguely straight, and put foam insulation between, depth varied 2 to 6 inches.
Then vapour barrier
Then ply lining

Also got a huge double glazed window off ebay for 99p :-) and added that that (facing the house, not visible from the street). Also bulked up doors with ply and angle iron for security and draughtproofed them.

Is now super dry and heats up quickly with an oil filled radiator if need be. Had no condensation probs at all, no probs at all leaving computers etc out there.

It all cost a bit but don't regret it at all. So much more pleasant working in a warm dry space. Also painted all of the ply lining white and added loads of lights - a bright workspace is good too.

Only regret is not doing the floor when it was empty. It's bare concrete, not very even or level - wish I'd dug it out/replaced, added insulation, levelled and would then probably have put down the industrial non slip vinyl with the gritty finish - there are sellers on ebay with roll ends going cheapish.

Well worth putting an effort in to make the garage a pleasant workspace before starting projects IMO. It might even add to house value (could be an 'artist's studio' or home office etc once insulated, wired up etc.)

Other things:
- MIG welding is dead easy to get the hang of. Might not be needed as you say, though, for a smallish build. Some people have been using epoxy and bolts to build steel frames e.g. MadVac CNC - home made 4'x8' cnc precision gantry router

- Drill press - would advise second hand here - I got an old English-made but rusty Meddings one from an autojumble for £60. Quality is a gazillion times better than most imports I've seen. Extend the quill all the way down then try wiggling it. Zero movement on the (40/50 years old?) Meddings, millimetres on the cheap new ones. Search ebay with results sorted by distance for "bench drill" and look for names like Elliott, Meddings, Pacera, Fobco. Prob best go for a single phase motor one to keep it simple for now. Also consider a floor standing model - often not much if any more pricey, can be handy sometimes and doesn't take up bench space. Random example of a bench one: bench pillar drill | eBay

- Drill sets - see drill sets | Model Engineer

- Other bits - good advice at e.g. Equipping a workshop | Model Engineer