Altima98

I own a '98 Nissan Altima 4 door sedan. Last year the " Service Engine Soon" light from the OBD turned ON. The Nissan dealer detected codes(P1448, P0440, P1447) for Evap Purge Flow Monitor. I was suggested to replace the bad canister & Vent Control Valve assembly with an estimate of around 450$( parts + Labor). I was also told that this being just an emissions issue should not have any problems with the driveability & performance of the car. So, I had the codes erased and postponed the maintenance. Exactly after one year, I found that the same MIL (Service Engine Soon) is back with the same codes detected. It keeps turning ON even after erasing the codes.
1. Does the problem need an immediate fix now?
2. What are the effects of driving the car with the MIL on/ bad canister & valve assembly?
3. Is the estimate of $ 450 for Canister & Valve Assembly reasonable?
4. Are there any cheaper alternatives?

I'd really appreciate help on this issue which is bugging me for the past one year.