Maintenance- same as a Glock. Doesn't need heavy lubrication or grease to function properly.
Parts replacements- Most likely unnecessary. The recoil spring is rated for a 20,000+ rounds service life and I have seen many USP's/USPc's go much longer round counts with zero parts replacements. If it doesn't look heavily used then chances are it is no where near the point of needing anything replaced.
I would buy a spare LEM TRS (trigger return spring) just to have one handy.
Night sights- I am fine with the Meprolight Tru-Dots that HK uses as their factory night sight option.
"Don't hit at all if it is honorably possible to avoid hitting; but never hit soft." ~ Theodore Roosevelt