Well, I don't recommend that you buy a new gel battery since it is best to have 3 batteries of the same type for a 36V motor. (Optima batteries are not gel batteries.) AGM and conventional cell batteries both have acid, so you're confused there too?
Your dilemma is between Optima AGM and conventional flooded cell. I don't think you can get much battery for $100 if you consider other charges when buying a battery. In addition to sales tax, isn't there a disposal fee even if you turn in an old battery as a core?
It will not hurt a new battery to use it with 2 other older AGM batteries for a 36V series connection as long as those other batteries remain serviceable. Disclaimer: this means that when either of the two older batteries starts to go, you replace that one too, or the new one will be more deeply discharged, which is not a good thing.