To go to RS232, you can use a MAX232 chip and a handful of small electrolytic capacitors, mounted on breadboard (for experimentation) or soldered into a small piece of veroboard (solderable breadboard.) That's less than $10 in parts. You'll probably pay more for the wire to hook it all up 
If $150 (Arduino + parts + pump + soldering iron + shipping) is not a bank-breaking amount for you, I say go for it -- sounds like a fun project!