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It's quite a broad set of questions you ask. I'll bite off one part, and that is the "entry level" model.
There are three inflection points in the normally aspirated part of the line, in my opinion. First, the 336/337 division. You get a lot more performance and an airframe that reflects improvements from early learning when you buy a '65 337 vs a '336. You also pay more, and have retractable gear expense/risk to deal with. Folks who read this board frequently know that I personally make this decision in favor of the 337, but there are many happy '336 owners that value things differently than I.
The second inflection point is 1968. There is a fairly rapid increase in gross weight from '65 to '68, from 4200 to 4400 lbs. The gross weight stays constant to '71, where it jumps again to 4630, but there are not very many '71 and '72 airplanes to choose from.
Up to and including 1972 you get a full six seat airplane, even if the fifth and six seats are really child seats.
From '73 on you get the fuselage that is shared with the P337, so you have the airstair door, which for a normally aspirated airplane might be a plus (ease of entry) or a minus (weight). If I recall correctly, you have only five seats in this airplane, but perhaps a '73+ normally aspirated owner can correct me on this. I know this is true of the P337. But '73 is the third inflection point in my opinion for normally aspirated because the interior changes, and you get electro-hydraulic gear rather than hydraulic, which I consider a plus because you don't have a "critical engine" (the one driving the gear pump). On the other hand (and there is always another hand in this discussion) a windmilling prop will retract the gear just fine, so all you really gain is a few seconds of delay before you can feather a prop. In 1973 you get a magneto access door on the rear firewall, which will make your mechanic very happy as well.
You might consider the larger gas tanks (148 gal) that started in 1975 and the sliding rear seat (1976) to be significant improvements worth noting as well.
You pays your money and you takes your choice.
Kevin
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