Well, ok now that you describe what you want to do with it I'm not so sure you are going to get what you want
The mods I've listed are a start point that would make the tank stiffer and stop the wheels falling off after 10 mins. I would (and do) run and battle with that kind set up on in a typical backyard type of area with long-ish grass and/or light dirt.
However, if you mean by
over come grass and heavy off road places
that you want it to go charging off thru open fields and into the woods like its some kind of off road buggy [X(] then I can can assure you that you will be dissapointed with a tank that cost 100 pounds. you will need to spend well over that ammount on the electronics alone as the the standard HL stuff will not take that kind of beating. A heavily modified tamiya tank is about the only thing that will help you there I'm afraid, and they aren't cheap.
Some more information would be useful here so that we can better help you
Mart
Quality tracks = Asiatam or Impact. Cheaper stuff is Heng Long - of all my tanks that have metal tracks and more than a couple of hours running on them, the only one that hasn't suffered loose track pins is the one running on an its impact set. The Heng Long stuff isn't necessaryly bad, just so long as you remember to watch for the pins working out.