The Sundowner and Sierra (as well as the Sport) all are descendants of the
Musketeer.
The 150HP Musketeer became the Sport.
The 180HP became the Sundowner.
The 200HP Mouse became the Sierra.
I think the entire Musketeer line is undervalued and provides some of the
best bang for the buck.
I have a 1966 A23-24 Super III, which was the 200HP, fixed gear Musketeer.
This is (IMHO) the best mix of the Sierra and Sundowner.
It has the IO-360. Great engine.
It is roooooomy. Mine has 6 seats. Some Sierras do too.
Useful load is about the same or more than a 172.
Mine is 960+ lbs. I burn 9.5 gal/hr and have 60 gallon tanks.

All my numbers seem to be about 5KTS faster than a 172 (I trained in a 172).
I rotate, cruise, and approach 5KTS faster.

The view out of the bubble windscreen is the best I've ever seen.
When straight and level, it looks like you're nose-down. Takes a while to
get used to.

I really like the trailing link gear. It makes all landings seem better
than they are ;-)

I've been so happy with mine, and I would highly recommend anyone to try one
out.
You might also check out the following type groups:

http://groups.yahoo.com/group/musketeermail
http://www.beechaeroclub.com

I haven't notices any large expenses to date.
I have done some mods, but they were Lycoming related, not Beech.
The planes are built like a rock.

Adam
N7966L
Beech Super III