The various Nautiluses (Nautili?) are all good tanks; I frequently recommend them for people wanting to move up out of the pro-tank arena. The basic difference between them is size and juice capacity. They also pair well with the iStick family. However, the iStick 50 is too much power for a Nauti, 30 watts is all you need to drive the available coils. If you want the additional power, or like the size, or the additional battery life in the iStick 50, I would recommend you pair it with the Kanger Subtank. It has +ohm, and subohm coils available, plus a RBA deck so custom coils can be built for it, including using TC wire if you are of such a mind and ever get that kind of mod (you don't have to build them yourself unless you want to learn how; you can pay your local B&M to coil it for you.) I don't have one, but I hear raves about the newer vertical coils wicked with organic cotton for the Subtank. An alternative to the iStick family in the same power ranges would be the Innokin iTazte MVP 3.0 and 3.0 Pro. Those are 30 and 60 watt mods with internal batteries. Yet another choice would the the Innokin iTazte Cool Fire 4 mod, a 40 watt box with an internal battery; another mod I don't have, but hear raves about. I personally think the Innokin products are a bit better built, a little higher quality, than the iSticks. But they are a few dollars more expensive. The Kanger Subox mod, a 50 watt model, deserves honorable mention in this list as well. However, the Subox requires the additional purchase of a removable 18650 battery (actually, you'd want at least two so one can be on the charger and the other in the mod) and a charger therefor. I personally think removable batteries are a plus rather than a drawback, but it does add expense and you have to futz with it more. This mod has the additional advantage of being paired with the Subtank in a starter kit, so you can save a couple dollars buying them together. A viable replacement for the Subtank would be the Innokin iTazte iSub G tank, but it will only work if you are certain you want to go subohm (I don't think you are, or not yet) as the only coils it has available are .5ohm, and it does not have a buildable deck. It's a few dollars cheaper than a Subtank.
If you are going to make the purchase, I think you should also take a serious look at a Temp Control system. There is now available the iStick 40 TC mod, which is just about identical to the 30, except 10 watts more powerful and had the TC circuits. They're brand new, nobody knows how well they will perform yet. Additionally, there is no Temp Control tank immediately available to pair it with from eLeaf (who makes the iSticks). The EVIC VT is a 60 watt TC mod with on-board batteries, and comes with a color matched tank with +ohm, subohm and TC coils available. (Most, I am tempted to say all, TC mods also work with regular old coils in variable wattage mode and thus will fire any tank.) Additionally, the Smok TCT is a temp control tank many people like. There are many options regarding TC, but all of them are larger, more powerful, more expensive (some a *lot* more expensive) and use removable batteries than these I have mentioned. However, I think TC is the coming thing; I really do think people in the market for a new mod should take a hard look at them, especially now some are available in a reasonable price range. I have recently bought two of them myself, a Snow Wolf and an SX Mini M. I can cheerfully recommend either, but I think both are bigger and more money than you're looking for. I'll add a photo of them, though, so you can see.
Your original thinking of the iStick/Nauti combo is not at all bad. It might be exactly what you want. But there are other options for near the same money; I recommend you look around a bit before deciding. Why don't you Google the gear I have told you about? If any of it catches your eye, give us a holler regarding your thinking, and we'll help you refine it a bit. Lol, or don't bother, and just buy an iStick 30 and a Nauti

Good luck and vape happy.