Are all these are SP maps?

yep, all sp. Spirit likes the singleplayer part.

there is a rating system for the maps/episodes.
for example i can remember the good map "zerstörer", was a good one.
http://www.quaddicted.com/reviews/zer.html

and its rating is "excellent". so that means you can trust the rating.

which engines do people generally use to play single player?
ill check a few out

so theres less-visual-tweaked engines for the original atmosphere, middle tweaked and heavy tweaked engines.

some older people prefer elemantary engines, with less new visuals, performance is not anymore important.

LordHavoc`s dark places is called the best modding engine on the market. he created a powerful engine.

last time i tried the ultra-highres textures from rygel. he works at the moment on crysis textures.
2,5 GB new textures for quake with dark places + dpmod, quake looks with these textures like doom3. a completely new game. problem is to find the most updated textures from him, because of the traffic and mirrors.
http://rygel.repage8.de/

i will ask him what to download at the moment.

I suggest Fitzquake for proper faithful Quake looks but a modern engine.

Also check out our http://www.quaddicted.com/quake-injector/ for easiest installing & playing. :)

downloaded darkplaces, has loads of options but can keep it authentic and still have much higher graphics and also runs great.

definately have to play some good old quake maps after the exams :-)