At level 45 I'm thinking you are going to have a hard time staring up RG, so I would go with ST Han. Some top arena teams in the current meta actually use both,so you wouldn't need to replace RG yet,but I'm also thinking that in a months time a 3 or 4 star RG might make your team a target in arena, by then you should have more options that you might want to replace RG for someone else. When you do, you wouldn't have to fill a void by not having a tank, you would already have Han.
As for who else to focus farming on, GS is a good easy to gear choice, Qui Gon is also very good. Lumi is probably going to be really useful in GW, but eventually less useful in arena, so maybe once you reach 6* switch to farming Phasma. She would make a great leader in arena. Also, try joining a guild, when you do you should focus on buying shards for Rey because she is probably the best dps in the game. Only Leia comes close to Rey, and that's mostly because her stealth makes her impossible to take out early in the fight.
