Well, everyone will have different opinions, and the price cap really puts a damper on the list. Otherwise I would be including things like the MP44, Thompson, BAR, etc. So, $400 or less, let me think...
Here's my ideas (excluding the M-N and Enfield as you have an example of each, but they are both very addicting):
1. M1 Garand--Love mine to death, just need more than one! Go CMP, and soon
2. M1 Carbine--Would love to get my hands on one, but they run about $450 and up; may just have to wait until my parents are in the ground to get my grandfather's
3. K31--Never owned/fired one, but they seem way cool, and everyone says they are uber-accurate.
4. Nice Mauser of some country and caliber. RC K98s are really cool, I like mine a lot. Swede Mauser would also be a nice choice, or (I utter the blasphemy) an 03 Springfield, since it is a very well executed copy/development of the Mauser design
5. And last but not least, a Krag. Preferrably a non-molested one, and if you were very lucky, a Norwegian in 6.5 Swede. I still kick myself for passing up on one 7 or 8 yrs ago. It was intact, minus sights and stock. I think the barrel may have been cut, but it was only $225, and I have never seen another one in person in my entire life. So, here's my brief list, with my suggestions.