Places to compromise:
Size too small - You can always adjust later
Price - If everything else is exactly what you want/need, nothing wrong with paying extra to get the bike
Consumables - Worry about the parts you'll be stuck with the longest, and compromise on the rest
Mechanical issues you know you can deal with - Perhaps someone tried an upgrade, gave up without finishing], and kept all the original parts to put it back
Rust - It's a rare bike that actually becomes too rusty to ride