In my opinion, your best choice would be the Boeing 737-200. However, if you can wait for a bit longer, you can aquire the Boeing 737-300 or Boeing 757-200.

I"'ll just wait for the classic B737 series (Now my 727's lifespan is only 70 months old)

No, don't do replace it with 737, it has lower speed, it could possibly reduce schedules and has lower capacity. The 757 has first class and it failed to fill up in my experiment making lesser money than 727. The 727 stops production in 1984, replace aging 727-200 with 727-200 temporarily and in 1988.1 you can replace all old 727 with A320 (I did this way). A320 has 180 seats while 727 has 189 and same cruise speed

But his 727s will reach their lifespans by 1986, too early for the A320.

Also i replaced all B727-200's with A300's (Really fills up) so i only need the B737 classics

Hmm, Interesting