So I noticed a guy touting a '68 Raleigh Grand Prix on CL for $170. I went to check it out and it's actually a late 70's model. (1976 is the best I can get from the pictures/decals). SN# NA6219175. I'm pretty excited as this is my first road/touring/commuting bike. Really pumped to learn.
I bought my 1972 Gran Prix when I worked in a Raleigh shop in 1973. They had several 1972 models left over that were discounted that suited my budget. The 1973 used Normandy hubs which were better than the old steel shell hubs they used in 1972. This probably still has the Normandy hubs.
This one you are looking at still has the old cottered crank like mine. I believe that the bottom bracket shell is not a common thread so you might not be able to install a different bottom bracket, so I hope you like the cottered crank because that might be the one you keep. If you rebuild the bottom bracket, add a dust shield so that particulate matter in the frame does not get into the bearings.
I did not know Brooks made a plastic seat at that time, mine has a Wright leather saddle.
Mine had simplex shifters and derailleurs, which I changed over to Suntour. I did not realize that later models used Huret. The Huret should work well for you.
The rear rack clamps on the seatstays, if you have weight on the rack and hit a hard bump, the clamp can slide down the stays and mess with the brakes. Put a loop of cord between the clamp and the brake cable bridge to prevent the rack clamp from slipping downward.
Looks like the stock 27 inch steel chromed rims, not very good in the rain.
From the photo I am not sure how many bolts are whitworth, but you will probably find that some (stem, seatpost clamp, maybe rear hub, cotter pins) are not metric, they are whitworth. I still have two whitworth wrenches (spanners to the purist) in my toolbox. I am not sure but I think that the fender eyelets are threaded for number 12 fine thread, which is virtually unheard of and impossible to buy. So, if you want to put new bolts in the fender eyelets you are likely looking at re-tapping the threads to something else.
I suspect this has 120mm rear hub spacing, if you change wheels consider having a shop stretch the frame out to 126mm for more gearing options. If you want to switch to 700c rims, one or both brakes may need to be replaced.
I still have my Gran Prix but quite frankly this was one notch above the lowest level of "10 speed" bike that they made. If you find a comparable vintage Raleigh with 531 tubing from their Carlton factory instead of their Nottingham factory, you might be happier.