Small-wheel folding bikes are among those bikes that have made tremendous advances. Further, good ones with 406 wheels have become commonplace, and lower in price, at least on the used market. Thus, I would recommend surveying the market and deciding what your long-term goals for the bike will be, and finding a nice used example as a canvas to paint on. What city do you live in? The markets vary greatly, a big bike city would offer better selection on craigslist.

I enjoy restoring old stuff. But in the case of a folding bike, the modern designs are so much better in performance.

An example of what I've done with a moderately cheap folder, a circa 2004 Dahon Speed 7; not as cheap as a Boardwalk, but has a frame that appears to be more torsionally stiff than the smaller-tube Boardwalk. Less expensive than a Mu.

Mods:
- Front and rear racks, rear that holds the panniers far enough aft for pedaling heel clearance.
- 2X gearing for 400% range (HUGE plus, I couldn't do the hills on the original 52x30 low), 21-85 gear inches on 50/34 x 11-30, plan to go 11-34 with next chain and cassette change. 2-piece hollow-spindle crank with external BB bearings. GS (mid) length rear derailleur with claw mount, as frame lacks a RD hanger.
- Improvised "Deltech" cable for better frame strength and stiffness (not shown in photo).
- Aero bars, bar ends on the flat bar, 2-position brake levers, mid-cable brake lever on left front of aero bar for front brake.
- All bearings rebuilt with careful preload adjustment, greatly improves durability.
- Wheels retrued to very close tolerance and with even spoke tensions, greatly improves durability.