I'd check out the proper HPI Website and see what you actually get for your money compared to the competitors such as the RAC. Last time I looked into this (4-5 years ago) HPI offered add ons and guarantees no one else did. At least to the trade anyway.
I have been stung when buying a car in the past with these cheaper HPI websites/apps they don't hold all of the information that HPI themselves own so it's easy to get caught out! Also there guarantees aren't worth the paper they are written on when it comes to claiming!
I only use HPI themselves now (RAC apparently uses the same info but I'm not 100%)
I wouldn't risk going cheap when buying an expensive car!