I think a good approach is to buy things if/when you need them, not in advance. Your example of a tig welder, wait until you have a specific project that requires it, and then look at your options. For fixing a toilet seal, only buy the parts you need, don't accumulate a 'spares' draw of parts you can pick up easily the day you need to work on something.
Maybe a friend buys a welder you could borrow, before you yourself need to use it. What if you need to unexpectedly fix a different problem? (roofing, tiling, etc...) Could you need another tool instead that you hadn't planned for?
I group tools with things like PC hardware, into "under $20", "~$50", "~$100", "~$250" and "~$500". I normally have $500(AUD) set aside for my next project (rather than a collection of tools of similar value), so its always there when I actually need something specific.
I've started working on a hardware MIDI controller with my little brother, so spent around $250(AUD) one afternoon on a soldering iron, arduino, book and a few bits and pieces (buttons, pots, LEDs). I avoided huge packs of LEDs, Resistors, etc... and just bought a few I knew I'd need (10k resistors for the buttons, etc...).
Anything left over, I'll donate or leave with him. If I don't have another HW project, I'll probably give him the soldering iron and multimeter too. Plus, I'll know who to ask first if I need to borrow one in the future ;)
I probably cost myself an additional $100 on this project, but I don't have to pay for a shed, or 'spare room' (like most of my friends have) to store various things, so it probably evens out over time.