The only problem with buying second hand bikes, is that they can be absolute lemons if they have been used but not been looked after.
I've bought four in the last year.
- One needed new wheels, chain and cassette
- One needed new tyres, bottom bracket, chain and cassette. And after I'd done all that and fitted mudguards, I found the frame had lost it's structural integrity. I'm not heavy, but just sitting on the bike flexed the seat and chain stays so much that the mudguard clearance, which was ample, disappeared.
- Another had a bent chainring, worn out rims and non-functioning brakes
- The one I bought via CycleChat, I'll say was okay but I had to inexpensively replace a shifter not long after I bought it
The latter was the best of the bunch and really well priced, so no slight against the forummer who sold it. More than happy with it. I felt compelled to pop it on the list for completeness sake - and as I get almost as much enjoyment out of fixing bikes as I do riding them, that's all good in my book.
My point is this. If you are buying a second hand bike, not from a shop or ideally local bike shop, then it's possible you are going to hit some issues.
I would also add that no bike is maintenance free. You either need to get on good terms with your local bike shop, or learn how to do some basic maintenance yourself. Otherwise a new bike soon ends up in a position where it's performing as well or as badly as the bike you are replacing. How much maintenance is needed depends on quite a few things - how often it's ridden in the rain, where it's ridden, how it's ridden, quality of the components etc..
The good news is that a lot of spares don't tend to be particularly expensive for these sorts of bikes and in forums like this, there is always someone on hand to help diagnose a problem and point you in the right direction when it comes to fixing it.
It might be better to 'fix' the bike you have, both the gears and the riding position. Some older mountain bikes, even with just a change of tyres, can be ideal for commuting, especially over a short distance. If you could post a picture of your bike, then we might be able to advise on the riding position? And what is it that you don't like about the riding position?
What sort of problems are you having with the gears at the moment?
Finally, if there are hills involved on your commute, then it would be good to get an understanding of what gear you are riding up them in. If you go for a new bike with a 1x drive train for instance or even a hub gear, then it's important to check that your new bike has an equivalent gear to allow you to get up that hill. So again, a picture of your bike is a good starting point for that, with perhaps a note about what front chain ring and rear cog you wind up in to ride up the hill - if you can remember.
Whatever you decide, good luck with it.
excellent value new or nearly new. Fits your criteria with adjustable bars and has a 1 x 8 drivetrain. Ive done thousands of miles on mine and can highly recommend. 
