Bench seats easy to fix - just ensure you use load spreaders as the metal isn't overly thick above the wheels.
Paddocks and others sell - haven't checked today, but memory says around £35 per bench
Have also seen a bench fitted with it's back against the bulkhead behind the drivers seat, but you need to put in some protection to stop the passenger and drivers heads meeting under sudden braking!
Fold up forward facing seats may be a better option and provides abiity to fit diagonal seat belt. Exmoor trim sell (pricey) but other options exist.
Ian (instantsquids) has done and posted photos if you search, or I'm sure would respond to a PM for advice
Also threads on here re: telling your insurance etc. re any mods made
PS - wouldn't refer to your Landy as a 'car' - it's inviting a ribbing. Jeep is also best avoided