It is possible to swap the nibs on all the flat top american or classic pens I've seen. The only difference is that instead of a metal grip section with a plastic nib holder screwed into it, and that holding the nib and feed together in place by friction, the classic pen is just a one piece plastic grip/holder with the feed and nib slipped in. To remove, just flush with warm water first, (unless you like ink on everything) then grip the nib and feed together just ahead of the end of the holder, and gently twist and rock while pulling straight out. Put the feed under the new nib, centered and with the right clearance from the tip to the feed, find the key or flat spot inside the holder, put the bottom of the feed on it and slide the nib and feed in together till they stop, then add a little pressure and rock side to side just a bit to seat firmly. Re ink and test, stop, adjust if needed, fiddle with it some more, write a couple of pages worth of something to make sure it is going to feed well. Advise the customer to take a brown paper bag, cut it open and write a page or two on the inside to polish the nib to his personal style of writing, and send him on his way.
If the customer is going to use the pen regularly, be prepared to get it back at some point when the plastic section snaps right at the top of the threads. It's a design flaw of that style and I've seen several do it.
For a kit in the price range you mention, check out the jr Gent at exoticblanks.com or your local rockler (it costs more there). The baron from arizonasilouette.com or navigator from woodcraft (same kit, different name) are also an option some like, but with some disadvantages IMHO.
