Congratulations, Gary, on your Mark V.
Others have given some great advice on our beloved brand's 12 strings, and what to watch for/expect in selecting an older guitar.
You may want to consider expanding your search to include the 70's as well as the 80's (of course, if you can find one from the 60's that is cool, too.) While not comprehensive, here is an idea of the 12 strings from the 70's and 80's (perhaps others can expound on/correct the listing):
Jumbo body shape:
F-512 (Rosewood back and sides, spruce top, G-shield headstock, block markers)
F-412 (Archback Maple back and sides, spruce top, G-shield headstock, block markers)
F-312 (Slightly smaller Rosewood back and sides, spruce top, Chesterfield headstock, side dot markers)
F-212XL (Mahogany back and sides, spruce top, Chesterfield headstock, side dot markers)
F-212 (Slightly smaller Mahogany back and sides, spruce top, Chesterfield headstock, side dot markers)
F-112 (Even smaller Mahogany back and sides, spruce top, Chesterfield headstock, side dot markers)
Dreadnought body shape:
G-312 (Rosewood back and sides, spruce top, Chesterfield headstock)
G-212 (Mahogany back and sides, spruce top, Chesterfield headstock)
D-312 (Rosewood back and sides, spruce top, Chesterfield headstock)
D-212 (Archback Mahogany back and sides, spruce top, Chesterfield headstock)
D-25/12 (replaced D-212)
D-225 (replaced D-25/12)
D-15/12 (Archback Mahogany back and sides, Mahogany top,Chesterfield headstock)
D-17/12 (Archback Mahogany back and sides, Mahogany top, Chesterfield headstock)
D-4/12 (Mahogany back and sides, Chesterfield headstock, dot markers)
The 80's saw the introduction of the "JF" and the "GF" (Gruhn) naming conventions, which takes some getting used to but added 12 strings to the mix, some having fewer appointments and some being new designs:
JF-65R/12 (F-512 "new and improved")
JF-65/12 (F-412)
JF-55/12 (replaced JF-65R/12)
JF-50/12 (Rosewood back and sides, spruce top, Chesterfield headstock, dot markers)
JF-30/12 (Archback Maple back and sides, spruce top, Chesterfield headstock, dot markers)
GF-50/12 (Slightly smaller Rosewood back and sides, spruce top, Chesterfield headstock (snakehead), dot markers)