Air conditioner in my 97 GMC Sierra Inop.?

Was blowing cool but not cold. Bought a recharge kit from napa psi. was a lil on the low side , boosted it to acceptable psi , was chilly for a coupe of days...Then nuttin just hot air like my mother-in law , mech. said I should have had the system evacuated and recharged, says he can prolly fix it for $140 by evacuating and replacing a seal and a valve , clean orafice & evaparator ,then recharging it (may not need the $700 compressor), says we should try this 1st, whadaya think ...oh yeah any sugestions on the mother-in-law would be greatly appreciated aswell .