Interesting, believe most collectors of the Lincoln series are in the same boat.
Of the 140 Lincoln cents (no varieties, double dies, or mint errors), I am down the 17 coins that don't exist in my set. The 14 D may never get upgraded, because of cost. Well over half are raw. Of the the very common coins, I have just not seen "the one" for my set. Of the 123 coins in my set, the average grade (raw and slabbed) is 63.5. The set is NOT red, but a mixture of BN, RB, and Red. Have looked at as many coins as possible to find the best at the most reasonable (NOT inflated pricing!!). I am close...very close. Good luck!
11 D (needs upgrade from AU55)
14 D (maybe upgrade from XF someday...maybe!)
14 S (raw, XF)
19 D (raw 63rb)
19 S (raw 62rb)
20 D need
20 S need
21 S (need upgrade from NGC 55)
23 S need
24 D (need upgrade AU55)
26 D (need upgrade AU55)
26 S (need upgrade AU58)
27 D (need upgrade AU55)
36 D need
45 D need
48 S need
50 need
51 S need
52 S need
55 D need
56 need
57 need
57 D need
58 D need