First off I would strongly recommend
VHO lighting on a 660 ballast, you can drive longer bulbs when you get the bigger tank.
If you are looking at getting a skimmer, dont go cheap, get one that will be adequate on a 125 right from the start
Most of the other stuff can be used as well, heaters ets, tho you might want to add another for the big tank.
If you use a sump now, you will prolly want a bigger return pump on the bigger tank but you could plan for an extra bulkhead fitting and use the one you have now on a closed loop, rather than adding more powerheads
