I am still looking into singlespeeds and by the end of the year (if i'm fit enough) I would like to convert my bike.

It's a 29er, with a shimano 7-speed rear and a typical 44-34-24 front.

The ratio I'm looking for is something decent for offroad riding, but also enough to get me to about 12-15 mph on the road without spinning out. My comfortable cadence is 70rpm or so right now, I might be able to get it up to 75-80 later on in the year but that's about as high as possible for me in sustained riding (not a spinner).

My offroad terrain would be mainly dirt roads, light trails,medium hills, some wind, some potholes, and very occasionally some proper off-roading.

Is it too much to ask for SS or is it reasonable? Also, a question about fixed gear- I like the benefit of trackstands, chain braking, etc, but I do need to coast on freewheel bikes now and again, as well as occasionally off-roading. Does fixed gear sort of lightly move your legs along or does it feel as though you're still pushing pedals?

And finally if I do take the plunge, what's a conversion kit going to cost me? This has to include the tool to remove the freewheel/cassette since i don't have bike-specific tools anymore. And what about a flip-flop hub, one side for road and one for off-road? My rear wheel is quickrelease and has a disc brake which i would like to keep, but I'm open to options.