Here is a great article on evaluating older Martins.
http://www.bryankimsey.com/70s_D28/index.htm
Byran has a lot of other information on his site about problems, what it costs to fix them and his theory on setup, action and mods. By the way, Bryan has done the neck resets and setups on both of mine.
If the ad is honest that could be a very nice guitar and sounds about like mine in its current condition. Having the neck reset is a good thing (that is the major problem with old Marties) - double check with a 24 inch straightedge (or the rule of thumb is that with acceptable 12th fret action you should have about 1/8 inch of saddle sticking out.
The pickguard crack is common and should be fixed. Sounds like yours was. Bone nut and saddle go along with a good setup - many people actually like the sound of stock plastic pins in a martie (but they get pretty beat up so you can decide). Fret wear is common - Bryan replaced the first 5 on mine and crowned the reset.
These guitars have non adjustable t/r's but usually have pretty stable necks. Both mine have about 0.010 relief after all these years - worth checking. A biggie is the saddle location - as one poster said some of them are in the wrong location. It can be fixed (fill the slot and reroute it) or if you play in just the first location you'll probably never notice.
Some of these guitars have been "hot rodded" (mine has and it is better for it). Kimsey is a great believer in replacing the bridgeplate, and did some after market scalloping on mine (my wife's comment when it came back was "you are playing louder tonight" - I wasn't). I did not have the popsicle popped - but some folks do.
Before doing the work mine was probably worth about $xxx (but unsaleble because of the neck angle). I've been offered xxxx for it (by a guy who owns a 1939 D-18 who doesn't like to take it out). I've also played it along side a GE and thought it held its own very nicely. fwiw, the work on my D-18 was about 700, on my D12-28 it was a bit over 6.
So, 1970's D-18's get a lot of grief, but there can be some real gems out there. Yours just might be one of them.