Classic 5* are dirt cheap these days. You can trade 4 level 40 4* units for a fresh 5*. The 5*s will give you enough firepower to rank or get higher BT points in events. And most importantly you will find recruiting stronger allies easier.
Getting a 6* will take a considerable time and a lot of effort for a free player. Panel events, such as the one going on now, can give out 6* rewards (Matahari and Ombre) just by grinding hard enough. Otherwise you need to accumulate TEs by selling 4* feeders, soul stones (during recruitment), and getting event prizes. The low end 6*s cost about 60~70 TE these days. You can expect them to drop further as 7*s roll in.