So here's the short version...

Do 20 paragraphs of RP before paying to get your vehicle. It costs different amounts at different places, and you can get your vehicle in various conditions.

To get it fixed up, do 20 paragraphs of RP, then pay for however much you're getting it fixed up. For every 5% of TRP you're getting fixed, you'll have to wait 1 day. Different prices at different places.

Same thing for modifications and detailings. Different prices at different places. 20 paragraphs of RP before getting detailing or modification done.

For more details and reasonings, please read the longer version of the rules.