Commando £ Values

If you're buying it to ride then it's worth what you, and others, are prepared to pay for it. I'm not going to say which are more desirable; it's either what you want or it isn't. Read the period magazine road tests that you'll find online. Spares situation is very good so there's no reason not to ride it.

If you're buying it as an investment then they're all over priced and I predict prices will come down to Dominator levels as owners find they can't ride them anymore and the kids don't want them. They made 60,000 of the things.

I would say buy from a friend or at least find a bike you know about. There are lots of stories about bikes being 'professionally restored' where it turns out they've been cobbled together from some box of old bits by someone who doesn't know a Whitworth bolt from something Screwfix sell.

Ashley Cutler was selling his, don't know if he still has it.
 

Mine is still for sale but I want to sell everything Norton in my garage and that is one MK3 and another MK3 project, both Inerstates.
Many 'restored' bikes are usually bodged together to get their money back or a profit. Alton kit may increase the price marginally, but make sure if the bike comes with the original parts. In the UK the Roadster is the more sought after as it is lighter to ride and move about. For touring and long distance the Interstate is the wanted bike. The difference being another 3 gallons of petrol sat high up on the bike. The fastback is no more expensive than the later bikes these days. 
If I was in the market for a Commando like you describe then the NOC is the place to look, the bikes tend to have long ownership and they are maintained by owners that tend to know what they are doing or know who to maintain the bike for them, most are used often so pretty reliable. So the bikes to aim for first time ownership I would suggest a late 750 Roadster or pre Mk3 850 Roadster for a right hand gear change or MK3 Roadster for the left hand gear change, but most owners swap from bikes with the gear change on either side with no issues. 

They are only worth what people are prepared to pay. Avoid a bike that has 'performance' parts fitted (like a hot camshaft etc). A bike with a single carb is better from a maintenance and economy point of view. An electric start bike (say with an Alton, CNW, or other kit) will be more expensive than a bike without and as Ashley says make sure it comes with the bits taken off. Ditto the carb swap. A single SU carbed roadster will have better MPG, so, similar range as a twin concentric interstate.
Be aware that an interstate when carrying a pillion is not a good handling bike. 
Brake wise, make sure that the disc brake has been upgraded to something that will cope with modern traffic. The original is crap.
Before buying anything ask a commando owner if you can start his bike, on the kickstart, Then imagine you are loaded up in traffic, stalled, in the middle of a busy junction. An electric start is now a godsend.
When you do find something, take someone with you that knows Commandos backwards, it might save you a lot of grief.
Avoid like the plague 'a just restored' bike. Ditto 'original' They will a nightmare of problems waiting to catch the unaware.         

I have been riding an 850 MkIII Interstate since new, probably 30% with a passenger, I have never had handling problems. Maybe I am just a little slower!

My recommendation would be exactly that, MkIII Interstate if you are intending to do long rides, Roadster if only short ones. But I may be a little biased.

on what you want.  The MK3 850 has a lot of issues on it sorted, the interstate has a longer range tank, single carbs seem to be more economical - but coming from Italian 70's bikes I think that this is not just a 'head' decision but a heart decision too.

The 750's were lighter weight, the roadster is very pretty, and the head angles a degree steeper, so for me they handle sweetly, sound great on peashooters, but moreover when I'm walking away from parking her up I always look over my shoulder...

Two up no worries - nice to be able to use up some of that torque!  

I was in the same position as you last year - I took a big risk on a bike at auction reasoning that I'd have some budget left over to 'put her right' if necessary.  4500 miles later she hasn't really missed a beat - just a few odd things to fix - but I've really enjoyed the fixing!