Its not that tough a diy job for anyone with a Haynes manual and patience. Its really just taking stuff apart and putting it back together. One thing to be extra careful with is putting the spring keepers back in, they are tiny and hard to manipulate with oily fingers and they tend to just disappear when you drop one. Only other thing that might require some finesse is lining up the cams, T-belt and distributor but since the 5m is non-interference motor, you won't break anything if you misalign, just move one tooth and try again. Special tools required are, valve spring compressor (~$25), a simple magnet fetch tool to remove spring keepers, 1/2" hose puller pliers (~$15) not required but make yanking the seals a piece-o-cake, torque wrench (~$25) and a 6' soft nylon rope 3/8" dia (feed rope through spark plug hole and turn crank till rope pressed against valves so they don't fall in). Make a weekend project and replace all the hidden heater hoses and all top end gaskets and seals while cam boxes and plenum are out of the way and mabe clean out the EGR too. Valve stem seal job is great opportunity to do a bunch of maintenance the car probably needs anyway.
Never had any problems with any aftermarket stem seals and I've done this quite a few times. Possibly slightly bent stem causing premature failure. Was failed seal ovaled upon removal?
Phil D.