Depends on what you want, if you want a bike with front suspension fork plan on spending at least $1,000 because cheaper bikes come with cheaper suspension forks that don't work good, they don't last long, and they're heavy. See:
https://www.amazon.com/BEIOU®-Mount..._UL160_SR160,160_&refRID=PBDC9N415WBFSD0HTF21
And see:
http://www.bikesdirect.com/products/motobecane/29er-mountain-bikes/fantom29_elite_xvi.htm
Those two bikes above are really good starter MTB's with suspension forks. If however you want something less expensive than those two then look only had rigid fork bikes, these are usually found on hybrid and cross bikes, not to many MTB's left anymore that don't have a suspension fork
You can read about the pros and cons of rigid and suspension fork bikes here:
https://blog.jensonusa.com/5-reasons-to-ride-a-rigid-bike/
Here is a nice rigid MTB for not a whole lot of money and it's got high reviews; see:
http://www.singletracks.com/bike-reviews/29er-bikes/Redline-Monocog-29er_1994
Full suspension bikes will cost twice what the others cost if you want reliable suspension parts and a lighter bike; this is probably, although not sure, the best for the money:
https://www.giant-bicycles.com/us/trance-advanced-27dot5-2-2016#specifications You can see how much the price jumped up from the others, and that's the lower end of the cost spectrum with half way decent suspension system. You can find cheaper full suspension bikes but like I mentioned earlier you get clunky suspensions with high failure rate that weigh a lot, not something real pleasant to ride trails on.
Please note, the bikes I listed aren't necessarily the best for the money, but should be darn close, they're may be other alternatives that others here will bring up that may be better or equal to to the ones I mentioned.
A swap may be difficult to find, but you could buy a MTB brand new and then sell your bike on CL; buying a used MTB could present problems if it was rode off road a lot, like blown suspension, wore out components, frame stress. Do note, having been married for over 35 years learned me a thing or two, selling a bike that your wife gave you might cause some hurt that she may not be willing to express to you, she may say that's ok, but deep down she might be hurt...though a few women may not care, you know your wife better than I do...I hope! LOL
Niall, it would be helpful and might get more replies if you posted in the "for sale" section of the forum stating where in the country you are and a picture or two might also help.
