Strictly personal opinions here...
1---I think you're better off buying a bike from a store (preferably a LBS or Dick's rather than WalMart, etc) than from an online vendor. The store will have it fully assembled, reasonably tuned and adjusted (hopefully maybe even reasonably fitted to you) and ready to roll for you, which is something that doesn't happen with the online vendors. Having your new bike assembled, RTR, at least semi-tuned and hopefully fitted to you saves you a lot of possible headaches and a probable visit to and expense @ a LBS to get the bike RTR.
2---Do some window-shopping at the LBS's. At least see where their bottom or low end is and what they have to say. They may have just the thing you need. You never know. If nothing else, they're almost always a good source of good advice and they'll let you do some test rides. Plus---they will be where you end up going for repairs/service, so you might as well meet them now, and... you'll get a better level of treatment (not to mention warranty service) if you buy your bike from them.
3---WalMart/K-Mart/Costco bikes tend to be disposable. They're cheap but they don't have repairable/replaceable components. When something goes wrong, you put the bike in a dumpster and buy another one, which gets costly. At the very least, go to Dick's Sporting Goods---they're a reasonable mid-point in the zone between WM/K-M/Costco on the bottom and the LBS's on the top end. The bottom-end big box store route is a roadmap to an inferior, uncomfortable bike and bad experiences.
4---You might be best off on a hybrid or a hardtail MTB.
5---I bought a Diamondback Sorrento around 8 years ago at Dicks. It's a hardtail MTB that cost me around $225 then and I think it's just over $300 today. I bought it to be my bad weather-winter "beater." My plan was to abuse it until it fell apart and then put it in a dumpster. That cheap bike has endured the worst riding conditions, it's had almost zero care/maintenance, it's served me without letup for almost a decade now and it's still going strong. I wish I could say the same for my more expensive, high-end bikes! You might find good end-of-season prices at Dicks in the second half of August and luck into a "refuses to die" Cheap-O MTB. If you do, swap out the stock seat for a comfortable one.
Whatever you do--good luck with your shopping and happy riding!