My 172H had a delightfully smooth six at the front, but it did suffer various costs I didn't expect. Because it hadn't flown enough when I bought it the gudgeon pins needed replacing (white metal in the filters). Not cheap. With six cylinders overhauls cost more, of course.
On early engines there was an AD on checking play in the square lug that connected oil pressure pump to camshaft. If it ends up round, no oil is pumped - very expensive. Make sure your engineer checks this.
If you go for a 172H and find you don't have enough power, once you've built up a (quite large) engine fund, you can convert everything firewall forward to 180HP Lycoming O-360 with a kit from Air Plains - see www.airplains.com
. It's virtually as cheap as having your engine zero-timed.
I did that 300 hours ago and the performance is better than a new 172 - because the airframe is lighter with more power. With the 40 degrees of flap you get on the older models you can get in and out of anywhere.
Good luck in your purchase!