If I were you I'd get the P-512 5.5in. bull barrel 22/45 or the P-4 4in. bull barrel. I like the magazine release on the 22/45 better (it more like a big pistol mag release) compared to the goofy little clip on the Mark II. Plastic or polymer is nothing new to the gun industry GLOCK's are polymer and they are carried by more law enforcement than any other pistol so it's proven.
You can look at www.gunsamerica.com or www.gunfinder.net to see if you can get a better deal or to see if your local dealer will match a deal that you find. I've seen a P-4 sell for $180 on some gun auction sites.