Honestly there's probably not a whole lot more to it than a portable heat pump. Obviously it would do heat, cool & air circulation, so the only real trick would be dehumidification. But that could be done pretty easily. Basically if you disconnect the exhaust pipe of a portable AC unit it becomes a dehumidifier. All you would need is a couple dampers to direct air either through the venting to the outside or back into the room if you want to dehumidify. The problem would be working out controls for everything. Not that it can't be done, or would even be necessarily that difficult. However given that about the cheapest I can find a portable heat pump is about $450 that doesn't leave you much budget to buy dampers & controls before it would become more expensive that the ACI one. However at $1,000+ you could maybe justify DIY'ing one.