I'll vouch for the M14 as well. Some have nice wood on the stocks, I refinished mine and it turned out very nice. Since then I've bedded it, modified the sights, unitized and shimmed to tighten the gas system, and built a stainless national-match style oprod guide from scratch. There are so many cheap ways to mess around with this rifle, you can definitely have a lot of fun with it. You can feed it good ammo and get decent groups, or you can use surplus to plink all day. Shooting these at gongs, etc. is a riot. As was already said, "boom... THWAP!". I could do that all day.
The cost of ammo is going to be the real price for a centerfire plinker. I bought a case of South African surplus for $550, that works out to 44 cents a round. You'll be paying about the same for .223. The only ones that beat this are the SKS or VZ58 at 18 cents/round. I own an SKS too and love it.