Questions about plastic rail from a newbie

Hope you're all well! I'll probably create a new thread for some of my specific questions, but I have been busy on ebay and have acquired a whole lot more track, rolling stock, and engines. Have yet to make any modifications yet - although I shall. Trying to figure out where to get boiler doors from that's cheaper than Hornby ones from eBay. Black modeling card, perhaps, although it isn't perfect.

A lot of the locomotives I've bought are very scruffy, though reliable runners. How can I best repaint them? Is there particular paint I'd be better off using?