03 was the first year that some SIG and probably the EXE had 4 sldes and 04 was the first year for the Dynasty. These three are all steel frame semi-mono-coupe on the RR10S chassis. Prevost/Newell/Foretravel all use a 10 airbag chassis but at a much higher cost. The SIG and Exe are 500+ HP and the Dynasty is the Cummins ISL 400 HP. The RR10S is noted for it's excellent handling and 9000+ lbs of CCC, but they tend to be heavy on the front axle. Around 06-07 Monaco started to move the fuel tank back from just behind the front axle.
I've owned my Dynasty since almost new and plan on using it for another 11+ years. If I had to replace it, I would try and find a 06-07 EXE with the Detroit 60 engine. I'm on the heavy side (45K lbs combined) and while I'm happy with my ISL 99% of the time, a little more HP would be nice. The barn doors on the basement came out in 06 and make it a little easier to get things out. Around 06 the SIG had a higher roof which would be nice if your are over 6'4" tall.
The more you learn and the more you keep up with prices you will know what's a bargain when you decide to buy. Good Luck