Pamestique 

Its unclear as to why your hands hurt and you can't handle the smaller seats... generally bigger seats are much more uncomfortable (more pressure points especially around the sitz bones).

I think your problem is getting a good fit on the bike. Really, its very hard to just buy a bike off the storeroom floor and it fits. Minor adjustments are always needed. For instance as to your hands, you may need a longer or shorter stem. In addition where your prior handlebars - flatbars? Maybe a good regular handlebar with hoods and drops are what you need. The point is there are many places to move your hands around during your ride so they don't hurt. If for some reason you want a mountain bike make sure the bike has "riser" v. flat. Also make sure the handlebars aren't too wide. That will place pressure on your wrists. The shop can trade them out.

As to the saddle - saddles are like shoes. It's possible the prior saddle was just not for you. It probably was not the width but the design.

So you gave us all little to no information. What's your price point? What type of trails are you riding? What is your projected distance? Do you have any interest in becoming a cyclist or do you just want something to roll 1/2 hour on a bike trail? Also what is your height? If over 5'9" then you can consider a 29er wheel but why aren't you then just not thinking about a 700c? Why the mountain bike? Are you riding dirt trails?

So really more info needed but bottom line go to your local bike shop and see what appeals to you. Your weight is not really an issue unless you are getting closer to 300 lbs or so...