I've got a vertical drop SS with no tensioner. I've only got maybe 200 miles on it so far (too many bikes to choose from) but I haven't had any problems yet. I know you'd have slightly more chain stretch with a fixed rather than a SS but...... Chains are what, $5? So $5 a month wouldn't be bad at all if you really liked the bike. 42-12 is a weird combo, but if 42-12 fits then 42-16 should fit as well and that's darn near a perfect gear. General rule is that you can go up or down in 4 tooth increments (total tooth count) and still have it fit, it's worked on my bike, your results may vary.
If it's cheap and rides nice then go for it, otherwise wait and get something that you KNOW will work better.