600 Medals are fine I would say, it's nothing major spend and you will probably still gain enough to have some shopping if you feel like it. Sadly, Nandi will probably not start until next week so you can level her on Sunday and then directly take on all the lower floors in one sitting, to make use of her.

One strategy to tackle tower is to do them on certain days, so for example, I will not do the first 2 floors until tomorrow and do the first 4 floors all together. Then I wait 3 days until Day 5 (Monday) and then do another set of 6 floors (reason is, I have no lockdowns on all my hime and can freely adjust). If you do that aswell, you have enough time to level Solomon and, if you really go in, also get her 15 MP MEX skill which resets her cooldowns, so you can make directly the most out of her. Last set of Floors would be on the final day, again, all lockdowns are lifted and you can readjust.