I want a comfortable fast ride. I like the more challenging trails, but have not quite gotten to all the reds. A lighter bike will give you the chance for more speed on cross country style trails that we have in this region. You are not talking downhill or aggressive drops like what is found in the Ocala free-ride area where you would want a slightly heavier bike.
So now the option is hardtail or full suspension. Hardtail allows more direct power transfer from your pedal stroke to drive train, but a well set-up full suspension will allow close to the same thing with the added comfort of not being quite as picky on the line you choose over roots, rocks, etc. On a hardtail you're just out of the saddle a bit more over those. I have both - I like my hardtail at places like Croom; the full suspension is more forgiving at like Alafia and Santos.
The other consideration is cost. There is a true definition in cycling from a $500 bike to a $1500 bike. Better quality in frame, components and the overall feel of the bike - as well as superior suspension as the price goes up. Buy as much as you can afford. Also from there you can always upgrade later.
I know my rear derailleur "cog" needs replacing. I have already had to have a part shaved down because it was bent and the shop did not have the part. Now it is not shifting well. Can you just replace that part or do I have to by a whole new derailleur? The deraillieur and cog are two different things. The chain goes over the separate cogs on the cassette attached to the rear wheel and runs through the deraillieur attached to the frame in the back. You have a front and rear deraillieur - the front shifts the chain from the smallest to biggest of three chain rings in the front. If the cassette had a cog that was bent likely the chain would need replacing too as a precaution, but unless the shop was able to align the cog exactly you will have shifting issues. Expect in the range of $25 for a good chain and $50/$75 for a LX quality cassette. (mid range but GOOD.)
Also, I was wondering about tire tread. Do they work like a cars, with a deeper tread gripping better? Tires you wil find almost as many opinions on as tires themselves. But here's the basics. For muddier conditions you want a little more space between the knobs, but doesn't necessarily have be big knob for Florida and most of what we see in the nearby states. Lower knob= lower rolling resistance. With that said I like a knob like the WTB Weirwolf for a great all condition tire. Runs about $50 each. Kenda Karma is a great tire with a low knob and spaced good for mud conditions too. I ride it on all trails in our area and like it. My hardtail has Kenda Kozmick Lite which is a slightly lower knob and closer together that works real well on everything except mud. There again in the $45/$50 range, but deals can be found.
I was looking for another bike because I was tinkering with the idea of riding the road. For that a descent base model would be what?? One bike that does both? Or another bike for road to compliment what you already have for off road. The "Fitness Bikes" that Giant, Trek and Specialized put out are great if you are looking to avoid a drop bar road bike. They offer much the same as you will find in a standard drop bar or racer model, but with mountain bars and shifters. They can start at around $5/600 and hit $1900/$2500. Depends on what you are looking for. An entry level road bike with quality components will start about $1200, like most mountain bikes. Obviously you can spend a whole lot more too.