Have you ever picked up and felt a race suit? Tried 1 on?
A friend and myself decided we were both going to buy a good suit because we were on the serious side of private trackdays and just wanted the extra safety that a suit provides. Our plan was to go to a local retailer, work out what we wanted then order it from overseas for about 25-30% discount (same brand, same sizes as Australian retailers).
I can tell you now that before buying my Sparco safety gear I went in to the Australian Distributor a dozen times before handing over any cash. I spent probably 2 hours trying on different model Sparco suits and I can tell you that just because they were the same size (number) they certainly weren't the same fit. Some were slightly longer in the legs but tighter around the belly, others were longer between under arm and hips, they all fit different between the various models even though they are made in the same factory and the same brand. Even after all that time spent trying on I got the suit home ($1,000) wore it around the house for a few hours over various nights just to get it to adjust and also to get used to the feeling of being in a suit like that and I still took it back and got a higher model that fit slightly better (Sparco X-Light $1,500).
The difference between the $400 "overalls" and a $100 proper suit is massive and then up again to the $1,500 suit was quite astonishing. The weight differences are amazing and the additional fire protection is also considerable.
Edit: The same can be said for helmets.
Buy the $150 job from a motorbike shop and you will get only $150 worth. The more expensive they get the lighter they are, the better ventilation they have and much better face screens on them.
It really comes down to, how much can you spend and the more you spend the safer and more comfortable everything will be.