98 12V
The '98 12v is a good truck. I have a '98 12v as well as an '01. They each have their own 'personalities' and both are great trucks.
As for the transmission, if you want a stick, then buy it. I wouldn't worry about the trans. The fifth gear nut comes off on some or maybe most of the transmissions, but it isn't a fatal failure. If it falls off, then you have a four speed trans untill you get it fixed. There are several ways to fix it. If you want additional info you should do a search of the forums for 'fifth gear' or '5th gear'. You will find plenty of reading. You are welcome to PM me with questions as well.
It seems that the trucks that towed fairly heavy and those which were lugged around at low rpm in 5th are the most likely to have the fifth gear nut issue.
The most important issue with the 12v engines and pre- '00 engines is the KDP, this can be a fatal failure. When I bought my '98 with 95K miles I was very nervous during the long 600 mile drive home thinking about that dowel pin ready to drop into the engine gear train. The very first thing I did when I got home with it was to let it cool off, pull the fan and front cover, and drive the dowel back in place [it was out about 1/8th inch] and peen it in place .
I would have more concern about the KDP than the transmission, check to see that there are no oil leaks on the front of the engine before you purchase. To repair an engine after the dowel pin has dropped into the gears can be over $3K. The KDP fix is relatively cheap.
Hope this helps, Greg L