Wow i had to read that twice to get it all, please don't turn this into another "there making a MKV Supra because i talked to a nobody at a Toyota dealership" thread
The backseats are big for a sports car with ass-trays, but there really not comfortable for anyone older than a teenager. Aftermarket seats won't save any leg room for the guy in back. I don't know if the MK3 is the best car for a beginner, it can definetly get you into trouble if you get to heavy on the throttle, but thats about the same as most sports cars. Yes a RB26, 25, 20, whatever skyline engine will fit into a 300, but unless you just have the money to throw away it's a waste. The VG30DeTT will make more power than you'll ever need. Theres really nothing to calculate, all the same era sports cars are roughly built the same althought some are alittle better than others in areas, but with the right suspension mods you can make one handle better than the other so it all comes down to which "style" you like the best, unless you just want to keep it stock or something.