If it were me, I'd buy a new set of 9-speed barcons, install the left shifter only and stay with 8-speed for the time being. Then, when other components start to go (cassette, chainrings) think more seriously about moving to 9-speed (and you'll already have the lever ready to use).

Most folks seem to agree that 8-speed is a bit more durable than 9, mainly due to the chain. However, lots of people are doing just fine with 9-speed setups (including me). Certainly, 8-speed cassettes will eventually be hard to find. Well, it already is hard to find a good quality 8-speed cassette; you are mainly relegated to Ebay for NOS cassettes (my MTB is still 8-speed, so I know they aren't all that easy to find).

To upgrade to 9 speed you'll need a new cassette and chain, of course, and possibly new chainrings (although if you stay with barcons you can probably get away without 9-speed rings).