The Merkava has been made in 3 versions. The Merkava 1 did not have the "convict clothing accessories". The 1st version is the one Tamiya produced several years ago. Whilst it is not up to today's Tamiya standards, I think it stills stands up well anyway.
I have built the Tamiya kit and would recommend it as a very good 2nd build. The Merkava Tank is bigger than it looks so don't be surprised when you look in the box.
There are heaps of after market out there for it, but I was happy with the OOTB result, with just a few simple improvements.
The latter Merkava 2 and Merkava 3 had the balls and chains, and kits of these have been produced by other kit builders-Acadamy being one. I have heard differing opinions about these kits, but most say you need to replace the B &C's as they are poorly done in the plastic. Both Legend and Royal models do a set of chains, and Legend does PE baskets for the Merkava 2 (and possibly a replacement turret).
I did see a built up Merkava 2 somewhere one the net, and the builder used the heads off seamstress pins and model chain to replace the kit product. It looked very impressive finished but I don't know if it scales out correctly.
As to your other questions:
1) The Tamiya Gun is fixed - the main gun has a dust (and sand) shroud affixed to it where it meets the turret, so moveable gun was not an option. You need to modify the kit parts to change the gun angle from horizontal.
2) Yes the tank does have an access door in the rear. It is a clamshell arrangement, and is used when the tank carries a few grunts inside like an APC. The Tank also has an airconditioner and water cooler for the troops too. (The Tamiya kit features no inside detail so if you want the back open, be prepared for some work, both research and plastic manipulation.
PS Good to see you've come over to the Darkside
Keep modelling Armour and you won't be disappointed.
Happy Modelling
Peter