The Contender is a strong action. It’s available in .375 Win, .45/70, .444 Mag, .223 Rem, and custom snot knockers like 6.5 & .375 JDJ. Better publications and web sites will list “Contender Only” loads.
It’s not indestructible, but it can be loaded a little hotter than the typical Remlin lever action.
A Contender with a 23” barrel will be about as long as a bolt action rifle with a 17 or 18” barrel, because the break-open design adds no action length. I have a 23” 7-30 carbine an it’s a short rifle to carry in the field.
What ever you choose use spire points and disregard SAMMI cartridge overall length. Load as long as your barrel's throat will allow, since you’re not cycling through an action. That extra length, and it can be significant, Contenders are notorious for long throats, means more volume in the case for powder. Which means more feet-per-second and no pressure increase on a properly worked-up load.
BTW, Boyd’s has their superb Contender thumbhole stock back in production, after several years of not being available. Highly recommended:
http://www.boydsgunstocks.com/product.htm?pid=20180&cat=1226
Don’t forget the matching forend.