Re: 20 x Nissan 180SX BPs
If you have gold, the best place to spend it is in Special Edition car BPs. These are available for a limited time, and once you are done with everything else you may feel this as a missed opportunity. Depends on how much gold you have, provided you need it for the Dodge Charger R/T event as well.
Another observation: 5 BPs costed 325 gold during the event, which means 1300 gold for 20 BPs. So this seems to be a discounted price.