As far as I know there was no difference in performance in any of the 240Z model years, depending on what accessories the car came with. If the car you found is in good shape - it'll be a joy to drive.
it is somewhat true. the 73 had the horrible flat top carburetors, weighed more and was slower. but if its in great shape you can always drop a L28 F54 block and an e88 head or a p90 head and be good to go.
Yes the '73 does weigh a little more , about 200 lbs., thicker sheetmetal . and yes you need to change the carbs to round top SUs . There are slight differences in the '73 from the '70------'72 but they are slight. If you are looking for a dragracer then that extra weight is a factor other wise I cant see any big difference. On any of the early Zs a good mod is to change to a electronic ignition . You can get one from the bone yard for about $50. If the car is clean and the price is right you cant miss. Gary Portland ore.