Having restored several Airheads myself and also being a V11 owner, I can make the following comments.
1. The bike pictured is a POS, most worrysome is the obvious rusting and Alu rust on the engine cases. Odds that the internals are in good working shape are low.
2. On the + side, BMW is VERY GOOD about parts availability for old bikes. That is why I switched over to BMW from Honda. When I needed to restore my Honda GL1200A, being more than 10 years old, Honda basically told me to pound sand for parts. Ebay helps, but there is only so far it will go. I have never ordered an old BMW part yet and been told NLA.
3. The V11 will literally run circles around an Airhead, mostly because the Airhead spends so many more miles wobbling as is goes down the road.
4. A well sorted Airhead is STONE reliable. Absolutely unbreakable. Easy to fix if you manage to break something. My Airhead R100GS is simply an awesome bike for touring and exploring in Mexico, it will never let you down.
If the motor turns and the gearbox shifts, maybe spend $250 on it. Expect to spend 3-4K and many sweat hours to restore. When done, it will be worth about the 3-4K you put into it.
Recommendation, many Airhead owners are past their riding days or dead. Look for a nicely maintained one being sold by a spouse who hasn't checked the KBB valuation.