I've had the same chromoly MTB for 20 years. I had test-driven every bike that was my size in the shop, and ended up getting the ugly duckling that was on sale ... because the frame geometry was perfect for me. Nowadays the frame is an ugly mess and I've replaced many parts but it's still my bike.
Always test ride before parting with your money. The length of the "top tube" is a secret ingredient that is rarely mentioned. You can adjust the seat and the handlebars to some degree but you cannot change the size of your torso and you cannot change the geometry of the frame so get one that fits. You will know instinctively - when you sit on it and reach over and grab the bars it will whisper one of the following: "your torso is too short", "your gonna do a nose-plant" or "you're ok kid!".
Find a bike that fits you perfectly and feels right. If you love it, you will love riding it, and will want to learn everything about it, including the things that don't interest you now.
What makes a new bike inexpensive is cheap components and construction, they look fine but they don't hold up to offroad use or abuse. A quality bike minimizes the chances for frustration, and a decent used one will cost about the same price as a new cheapie. I am not up to date on brands or what's currently available, however a quick search on the Navigator yielded a nice selection
http://craiglook.com/all.html?q=trek+navigator
Keep an eye on craigslist and your local classifieds.
Good luck! Feel free to PM me.