I'd agree with what LWaB says. I'd also add that Moultons are an unusual UK made bike which is something that has both good and bad aspects to it.
I've done a lot of riding on various bikes with suspension and yet I still find myself going more slowly on my Moulton than I'd like/expect.
There is more than one way to skin a cat; amongst my bikes is soft-tail MTB. It is about 10lbs lighter than my moulton, is every bit as comfortable, and probably it is (with the right tyres fitted) as quick too. A full suspension MTB I have is also lighter than the Moulton, and has a suspension that can be adjusted more. Neither machine would be a quick into a headwind on the road as a Moulton with dropped bars; partly that is the riding position and partly the bike itself.
I recently spent a little while riding a bike with a suspension seat post. They are fairly horrid things as a rule but this one had a special feature; it was noisy as it moved. The interesting thing was that it did move at various times, not always when I expected it. Suspension seat posts can be set so that they don't move at all during normal pedalling (which is good because it is an interference) but they can still take the sting out of big bumps. There are also such things as suspension stems, which can work OK with flat bars and even (if they have a parallelogram linkage) with dropped bars.
The interesting thing is that with a proper suspension arrangement, even when you are pretty beaten up, it isn't as fatiguing to carry on riding (esp on surfaces that are less than perfect) as it is on a normal bike. You can just sit in the saddle ( ahem..."like a sack of s**t" as some folk put it) and twiddle away. It is difficult to describe this sensation until you have experienced it for yourself.
However by their very nature, such machines, if they offer the best isolation from the vibrations of the road, are not best suited to out of the saddle efforts etc and may not climb well either unless you can turn the suspension off in some way. Bobbing whilst climbing is in good part due to the effects of chain tension on suspension movement. Here I'd have to say that I think the Moulton is at a disadvantage to some other suspension designs.
It is as well to remember that AM decided that his bike could have small wheels provided it also had suspension. The small wheels fall into holes into the road more than larger wheels so 'every road is rougher' for small wheels. That it can end up more comfortable than a rigid bike with large wheels is bonus/accident.
If you are regularly riding on rough surfaces, the tyres that work best on the road might not be tough enough for the rough bits. I have used fatter tyres (such as big apples) on my Moulton and the net result is (IMHO) distinctly sluggish. Tyres with heavy treads are also sluggish when fitted to Moultons.
Also do bear in mind that you are left with some of the other side-effects of small wheels, specifically
- handling; I don't find it at all easy to ride no hands with small wheels and even after prolonged use I'd say I'm more likely not to follow a wheel well and more likely not to follow my chosen line whilst looking over my shoulder
- acceleration; it is slightly better on small wheels but by calculation it is only worth about 1.5% to 2% on a machine with 20" wheels vs 700C (with equivalent wheel rims/tyres)
- gearing; getting enough low gears is fairly easy but getting good medium/high gearing can be problematic. You may find yourself having to buy large (expensive) chainrings that don't shift especially well, and using small sprockets too. The small ones that are used for tapping along tend to wear out faster than normal and the small ones that are needed for a high top gear can also cost a small fortune.
- the rear derailleur is that bit closer to terra firma which means it can be clouted by rocks, kerbs etc more easily and will (all things being equal) get a bit dirtier too.
So I'd suggest that trying a machine out and seeing how you get on is a very good idea.
cheers