1. How good really is the MP experience, is it buggy stable etc?
2. How many still play online?
if you have a KM1 install (Kobayashi Maru mod, which basically contains the best and most updated mods out there, in an all-in-one package, and has all the mods working together perfectly well in harmony), that mod is the best thing for MP use, and alot of people play as such... BC's MP realm is still very much active, tho i personally dont play MP...
3. How demanding is this game with the most popular mods installed? My spec is AMD 4200x2, 7800GTX, 2GB Ram, will this run the game on full settings?
4. Can i run 1680x1050 aspect?
that set-up should be perfectly fine for BC... you should be able to experience very smooth gameplay with several ships and such with all settings on high...
unfortunately, BC cannot support widescreen (yet) which is a shame... when BC was manufactured, widescreen wasnt very popular or widespread yet...
5. Are there many mods that add extra mission etc in the SP game, basically mods expanding the SP experience or are most of the mods cosmetic?
there is a mega-mod called Deep Space Nine FX, which comes with KM1, that contains mini-missions within it... there is also a mod called Quick Battle Replacement (QBR, also comes with KM1) which is like an "expanded" version of QB and there are many missions available to download, or you can also create your own...
as far as Single Player goes, that is the trickiest and most complex aspect of BC that hasnt been delved into much...
if your interest is playing Single Player, you would be best going with an install that is modded with BC Supermod 3.0 (as suggested) and nothing else installed... BC Supermod wasnt designed to work with any other mods installed and was designed to work only as it is...
you could always have a second BC install containing KM1 that could be used for MP use; virtually everyone who plays MP uses a KM1 install... however, for MP use, you cannot mod that install since MP requires all parties involved to have the same exact scripts to work... also, SP wont work well with KM, unfortunately...
of course, you could have a third install for your own use to modify as much as youd like...
I know ?30 is not the earth but i am only looking to buy one PC game at the moment and it needs to last me a while.
At the moment its a toss up between this and Left 4 Dead.
thats actually not too bad... ive seen it go for much much more... and if you do decide to purchase it, it should keep you rather busy for quite a while with all the ships, bridges, mods, etc etc etc to try out and play around with...
and of course, we're always here if you ever do have any questions about anything BC-related; dont ever hesitate to ask...