Go Series 2 with full OBDII (i.e., 97+, preferably 98+). Aftermarket for parts is huge (as has been mentioned), and you can tune it with a tuner (if you decide to go that far).

Plus, by 98+ GM had gotten rid of the ugly, old-fashioned, flat black side body moldings on Buicks.

Just found this thread and I believe I am qualified to answer :P

Plain and simple what are you looking for

S1 L67 in a Riviera is better to get the car moving because of the lower torque peak but it runs out of breath at about 85 MPH (stop light racer)

S2 L67 in a Riviera is more midrange power with a higher RPM torque peak. But it still has breath until triple digit speeds. (1/4 runner)

question at the end is how much are you going to mod it.

One point I want to make is the Riviera is the slowest of the L67 cars due to its weight. Keep that in mind while you are shopping.