'94 Impreza a/c question

I am buying a '94 Impreza, and it is in pretty good shape except
for being a bit rusty, and the a/c does not cool.

It appears to be R12 and not converted, and when I checked
the fuse it is OK, the fans start when the a/c is switched on
and it appears to have some R12 in it because when I quickly
pushed on the little valve in the fill fitting I got a good squirt
which seemed to have some pressure behind it - but the compressor does not start, though it spins freely by hand.

I also pulled all 4 a/c relays and switched them around with
no change evident.

Questions:
1 - Is there any reason NOT to convert it to R134a ?

2 - Assuming it is low on gas and that's why it won't run,
when/if I convert it, is there anything special I must do ?

3 - Is there another common cause for these symptoms ?

4 - Where are the high and low pressure switches located ?

5 - Is there an online source of repair info for these cars ?
(Maybe like Honda's online manuals ?)

Any education would be greatly appreciated !

mark