I have had both 4 and 5 speed boxes in different cars with the 4 speed one being my main car for quite a while back in 80's.
Certainly for blatting around the lanes now with the 5 speed, I spend most of the time in 3rd and 4th but even so when you get stuck behind the old dears on the single track roads around the countryside, 5th is a bit more relaxed!
Likewise on the motorway, it drops the revs just enough to be less buzzy (technical term, there!) and a bit more suited to cruising along...
Not sure about the chassis mods but the work to take the engine out and replace the box is very straightforward stuff. If the price is right for the car, I'd buy it and then think about any changeover after you have had chance to play... and that could be quite a while!
Nick