Does it not leave me future-proof when 18" drivers are more readily available and maybe I want to upgrade?
How much of a hurry are you in to build a sub now? In my opinion, and perhaps its just mine, the chosen amp should be considered a part of the design, so if you plan on going with a totally different design later on, odds are that you would be better off going with a different amp. However, dual HiFi 15 LLTs won't be a slouch, and even if 18" drivers become available, you'd be hard pressed to beat the performance/dollar ratio of a pair of these subs.
One note, as Thomas mentioned it recently and I think it may have went unnoticed. The HiFi15 was basically assumed to be suitable for downfiring - after doing the calculations, cone sag will be slightly more than what is typically the allowable limit, basically 5.5% when 5% is the typical allowance. I don't know how this might affect your plans.