The 325i is quieter than the 320d?
How much quieter, I found my 320d pretty quiet in the cabin. The insulation and sound deadening is pretty darn good in the E90.
I could pump my music pretty loud and be on party mode inside the car and not hear much outside the car with sunroof down and windows up - this was with aftermarket sound and an IB subwoofer install. No shitty rattles either.
If you getting the 320d checked out, then take it to one of your mechanics and ask them to check specific things as well - as mentioned in this thread.
Also try take it to ZF to have the gearbox checked out, the current mileage is likely too high to do a gearbox service and ZF will also be able to give an indication as to whether the condition of the gearbox matches the mileage stated on the car. A gearbox refurbishment is around R40k (maybe a bit more now).
Also try confirm whether the car is stock and find out why the exhaust was changed as the exhaust tips are not stock. The E90 320d has an exhaust which points downwards as per the below.