What's the difference between the Mark II and Mark III's? Is $450 a good price for a Mark II?

I think $450 is a good price.

http://www.hipowersandhandguns.com/DifferenceMkIIandMkIII.htm

Short version:

Sights dovetailed into slide with MKIII (MKII, the front sight wasn't dovetailed in)
Introduction of .40S&W to the platform - early frames were forged, later frames were cast as the forged frames weren't strong enough to withstand the repeated pounding of 40S&W - yes, cast frames are more desirable here, longer service life
Firing pin/drop safety added

...that's about it, I think...

$450 bucks for a Mk II, you better fo and fo hard

The feed ramp on the MKIII is also better for feeding JHP ammo than the MKII.

Stephen Camp was a nice guy. Shame he died.

Both have the straight feed ramp to reliably feed JHPs, but the MkIII has a loaded chamber indicator as well on the barrel. Different ejection port (changes on the MkIII to deal with cracking from shooting a bunch of SMG ammo, IIRC). Front sight on the MkII is integral to the slide as is the rib that it is a part of; it can't be changed. MkIII uses a dovetail (the classic used a staked and soldered sight). The MkII used the older, smaller rear dovetail but with a better sight than the classic. The MkIII rear sight is better. MkII has the extended bushing, the MkIII does not. Most MkIIIs have cast frames; all MkIIs are forged.

Most MkIII pistols are actually the MkIIIS. Those have a firing pin safety. The Israeli MkIII imports are usually the MkIII proper and don't have it. There is also a MkIIS, which is a MkII with the firing pin safety, which came out later during MkII production. Israeli MkIIIs also have a rib on the bottom of the frame.

There are also hybrids that have features of the classic, MkII, and/or MkIII. Some MkII and MkIII pistols were also made with older features, like the old-style safety, the ring hammer, lanyard rings, and the older style plastic or wood stocks.

these threads always cause me to look at buying a Hi Power Mark III