spectrapure Ro/di

Hi everyone well in my quest to get my tank set up and going I had to cut cost here and there and one of those was a cheaper ro/di system. $65ish off amazon. Well I believe in testing everything so got a tds meter and believe it or not was getting 0 out of it for the first 150 gals of filtered water at that point it slowly started rising. Well luckily amazon has an awesome return policy so now I’m back on the hunt for an ro/di unit. I know I’m going to go with a spectrapure and I’ve been told the refurbished are as good as new. But I’ve been offered a older unit for $50 but it 60 gpd I would prefer 90 gpd. Now just looking at the pictures both membranes look the same so I was wondering if there interchangeable? Or do I need to call spectrapure. I’m also wondering if I spend $50 on the unit $40 on a new membrane and probably new di media if I’ll end up spending more then just buying new. I found the spectrapure 90 new on one site for $155 with free shipping might go that route. It’s pretty sad that buying a used unit and new filters cost this close to the same. Any suggestions