I had to fix the wiper system. The wiper system you would think it wouldn't matter much whether the parts are aftermarket or not. I was very wrong, parts that even look almost identical may not work properly, due to differences in tolerances.
There is also different revisions of particular parts and it will become obsolete. You can lose an afternoon on the internet just doing that.
Then there is the tools. I've spent about a small fortune on tools. I have 3 torque wrenches, 3 sets of sockets, 3 sets of spanners and loads of weird specialist tools like special pliers. There are many jobs I can't do myself because they needs specialist knowledge to do properly e.g. gearboxes.
You have to be prepared to spend potentially years on it and huge amount of money, even on relatively simple projects.
There is a reason that a lot of guys get into old 4x4 pickups and do those up, because they are a known quantity and parts are readily available.