How do I quickly build an affordable Danish folk costume (circa 1750-1850)? 

Danish folk costumes are incredibly diverse, storied and beautiful. With so many famous regional variants (e.g., Ringkobing, Amager, Falster, Drejø, Rømø, Fanø, Læsø, Røsnæs and Hedebo, to name a few) Denmark's rich tapestry of folk costumes cannot be summarized or compressed into one outfit. That said, if you're looking for an affordable and easy outfit that screams Denmark (not Norway or Sweden), consider the following.

For ladies:

Long-sleeve white chemise (shirt)

Red nattrøje (a snug-fitting red knit sweater with a pattern; see photos) - optional: add forest green silk ribbon to the sweater sleeves for a Roskilde, Køge, Tåsinge Island or Lolland Island look.

Long skirt (solid or vertical stripes)

Optional: bodice (vest; see photos) - this is probably the hardest item to find or have made. Here's a Facebook page for buying Scandinavian folk clothing: https://www.facebook.com/groups/287096172036026/

Optional: apron (see photos)

For gentlemen:

Long-sleeve loose white shirt

Vest (solid red or vertical stripes; see photos)

Long black pants (worn after 1800)
optional: for the 1750-1800 look, www.Townsends.us sells fall-front breeches and stockings (socks), as do www.norskflid.no/bunad/herre/ and www.samsonhistorical.com/