The low side pressure can be from a clogged expansion valve too, but I'd say you are low on charge.
However, if you have a leak, I'd get the R12 recovered and fix the leak rather than just add more, you really need to evacuate the system if it has been leaking, since air will be diffusing in while the freon is leaking out.
The switch for the aux fan can be either pressure or temp -- don't know which one yours is. I don't get aux fan operation on the 300TE until 300 psi or so, usually.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!