if you are talking about building a stand-alone HERMS vessle (not a coil in the HLT), with its own heater, and reusing that in a later, larger setup- sure that would work.
and most of the stuff with the exception of the kettles, heating elements, and herms coil should be able to be moved over to a bigger system
those exceptions that are not reusable happen to be the most expensive parts. i would highly suggest building only one system and doing it in the correct size the first time. you will be spending almost as much money buying a smaller setup as you will the larger one, so unless you really want to buy everything twice, i suggest waiting.
if you want to start now, either build a stand-alone HERMS unit (not integrated with the HLT) which will work with either setup, or build your full size HLT+HERMS now so that you dont have to throw everything away later. a 10 or 15 gallon HLT will work for 2.5gal batches.
i would also use a physically smaller element than the one linked (assuming the picture is the actual product, there is zero info on that site). there are 2kW high density elements that are only 8" long and not coiled over like that one. simply so that it doesnt get in the way in the HLT. you only need low watt-density if you are boiling wort with it. if its just water, you arent in danger of scorching anything.
also- these guys have the cheapest camlocks you will find