Well let's look at the current system. Leveling from 1-60 takes nearly no time anymore, you can accomplish that in a couple of days, if you put enough time and effort into it. 60-70 takes a week, maybe two if you are not using the pvp method as only reliable source of exp and are watching the cut scenes and reading the dialog.
MSQ on the other hand from 1-50 takes not much time either, but then comes the MSQ hell gate, that you will have to unlock just to reach the heavensward area's and their new jobs if you're interested in that. This alone are 250+ quests, then the whole HW MSQ, we're looking at roughly 500+ quests total for both ARR and HW before even reaching SB content. SB's MSQ is another bunch of ultra long quests.
Also I do know that they did say 1 potion per account, I am not sure if that was just for the job potion, or also includes the MSQ. If that's the case, you would only be able to use one or the other. Might be wrong, however even if that's not the case, my suggestion if you do not want to dish out $50 or even more, just level via potd to 60, your friends can even help you with that since speed runs take only couple of minutes. And then spend your money on the MSQ potions for ARR and HW, I think the HW one includes the unlock of ARR and you wouldn't need to purchase both of them.
Edit: If you take your time and do some research, I've posted many comments on the other topics about that, and then exactly know what you're doing and skipping everything unnecessary in the game, including lore and cutscene's, and have 4+ hours a day on your hand, you will be able to reach 1-60 in just 7 days. Plus or minus a day, give or take. That's when running regular content, aka MSQ for a little bit, dungeons, hunting logs, Clan hunts starting with HW, daily roulette's.
If you're leveling via palace of the dead, the first 50 levels can be very slow, since you don't have another class/job at higher level. Meaning you won't get the armoury bonus that you would be getting if that's the case. However in circumstances of being DPS as example, PoTD can be faster since dungeon queue's take forever and even longer now with the expansion being released. As tank and heals you nearly get instant queue therefore, you are faster doing regular dungeons, as long as you chose dungeons at your level or one/two levels lower. Having done it on an alt, I can tell you the first 60 levels take around a week and half, maybe two. Now difference would be, if you're spending more then 8 hours a day in POTD with your friends, making runs quicker and just speed running these, still will take a good while, in comparison to investing that kind of time to the regular content, then you would be 60 within 3-5 days.
But if you don't want to go that route, then after you checked if it's possible to purchase both, that would be your only way to be able to access the current SB content as of right now. Don't make the mistake and buy the MSQ unlock before you're 60, since it opens a whole other can of problems lol