I tried to post this message earlier, but it didn't go through, so will retry.
My first inflatable was a 1977 Mark 1c which my brother and I bought used in the mid 80's we used that boat hard for over 20 years. Over that time had replaced a floorboard or 2. Within the last 10 years we had the thrustboard, floor fabric and most recently the transom reglued. About 4 years ago, I had decided it was time for a newer larger boat and ordered a Gemini Dive 470. While waiting delivery for it (about 1 year from the time of order), a Mark II Grand Raid showed up on Ebay at a great price (under $ 1000 CDN) so I snapped it up. So I'm now very familiar with the Mark I & Mark II zodiacs as well as a 4.7m (similar size as a Mark III).
The Mark I was a lot of fun but required some careful planning and packing if 2 people wanted to bring basic gear and food for a 24 day camping trip. It also mean't sitting on top of a mound of gear, which wasn't terribly comfortable. The Mark II does give considerably more room for gear. With these SIBs with an inflatable center keel, you will get pounded if trying to maintain a planing speed while in choppy conditions. With a 4.2m Mark II the pounding won't be quite as severe as with a Mark I, but it will still be severe just the same.
A tunnel hull SIB or a light weight RIB (of similar weight to a Mark II) in the 4.2m length range will reduce that pounding considerably, as well as be more fuel efficient and have a higher top speed (for a given motor). If you do have space for a trailer, a RIB is worth considering.
Regardless of what boat you decide one, I'd still hang on to that Mark I. With a sturdy set of launching wheels and a ~6 hp outboard, you could had pul that boat all over the back country to portages to get at those smaller lakes.