Brakes it the #1 thing. It doesn't matter if it goes, if it doesn't stop you're in for a bad day (& yes, I speak from expereince!). I did a full rebuild, all new lines, hoses, master cylender, & front slaves, and then I rebuilt the rear slaves and the parking brake.
As was said before, if you have split rims take them to a truck or farm equipment shop, most common tire places flat out prohibit working on splits. I lucked out and the truck tire shop at a local chain still had a couple older guys that knew what to do with them, & I hear they had a lot of fun teaching the young guys how it was done back in the bad 'ol days.
Wiring is a definate must do at some time, if you plan on making it 12V, may as well!
As for what will cost more, a child, wife, or truck,... Well, it may cost you your wife, but otherwise they can be a pretty even split. If you have the time to look for parts & can get a good deal on them it's not too bad at all.
