id confirm the kays of both the engine and chassis. the dash cluster bolts are surrounded with black foam when its installed, so check that to confirm that the chassis k's are accurate. (foam has to be removed to properly remove the cluster).
ask for receipts for the engine, and try to confirm mileage again, 20B into any rx isnt a huge job, but it isnt particularly regency friendly, look to see if the brakes and suspension have been upgraded to suit the weight and additional power of the 20B. (remember, a 20B is more than 50% heavier than the original 13B). if the engine is getting close to 100,000km, prepare for a rebuild (approx $5-7k). oil consumption and rotors go hand in hand, so dont worry too much, but as usual, if its blowing oil smoke, avoid it. also never use semi/full synthetic oil in it, only use mineral oils and mazda rotory oil.
pivot balls in the s5 boxes tend to be the weak point when youre using a heavy duty clutch (which you would be with a 20B-TT conversion), but replace those with a billet item and the box will last forever.
also keep in mind that a rotor will never pass an emissions test.
and if he does pick it up, tell him to be very particular with his startup and warm down procedures, start it up, let it idle in the drive way for a minute, and when driving, dont rev it any higher than 2,000rpm until youre up to operating temps, when youre finished driving, let it idle for a minute or 2 or 5 depending how hard youve been driving before shutting it down.
try not to drive it short distances, always allow it to get up to operating temps before shutting it down, starting it and shutting it down in small time frame and you run the risk of flooding the engine which can be a huge pain in the ass to clear.
also, take it to a reputable rotary workshop before purchase (eg, REVS or Mildrens) and have them look over the car with a fine tooth comb and throw it on the dyno.
happy motoring.