If you don't know whether you need it and he's not giving you a VERY good deal to get it now, why not wait and see. If you feel it's necessary then you can get the external unit (which may be a better bet anyway)
When I went to see them I didn't know too much about this lark, and put my trust in them. Whilst I think they were fair there's stuff I got that I'll never use and let's face it they are in business to flog you stuff.
Didn't Cleaning company have his set up for sale recently? He's emmegrating and he has a internal v2 400psi in article for sale.