The intent is for the Marquee to give enough material and currency to catch up to the free-to-play progression. You are intended to clear Tier 1 with just Luke, Tier 2 with Luke and IG-90, Tier 3 with them and Choda, and Tier 4 with all them and Asajj. They should all be around level 45 when that happens. The Marquee for Barriss puts her at 45 so the team can do Tier 5. That’s why Asajj was 35, Choda was 25, and IG-90 was 15.
If you buy with money or crystals, you jump ahead of that progression. You will not only have to pay for the older characters to get say 20 levels ahead, but the new characters would still need mats and currency purchased for them if the rest of the team is ahead of their Marquee starting point.
My understanding of Era Currency is that it is permanent. You have it until you spend it on Relic 10 or the LSTs. The Era Level Currency and the materials all go away at the end. I believe they are converted into just regular credits. So you can’t buy an advantage for the future Era. Whatever you buy prior to Mark 7 is just to get ahead of the free-to-play curve and you will end up with more than you can use, which becomes credits when it’s over. Definitely be careful buying Level Currency and only get enough to use the materials you have. Stockpiling does no good.
