Is it worth more to upgrade my 13" macbook pro mid 2012 or to buy a new one?

I've been considering buying one of the new mac models for better performance but friends and various sources online have all recommended I instead upgrade my mac for a fraction of the price. The fan in my mac died earlier this year and I replaced it myself with no issue so I already have the tools and understanding to open my mac and with some care not damage it in the process.

I make music with Logic Pro X, and I was experiencing problems with it, system overload messages all the time, but now I use an external soundcard which seems to have made Logic run much better with rare issues. Aside from this though my mac is still very slow sometimes. It can take 5-10 minutes to boot up, and sometimes it will boot with a screen full of static and is unresponsive, a restart fixes this but it's a little unnerving. Also the battery needs urgent replacing but I'm not too worried about it for now. The screen has a very thin crack in it from an accidental drop one year ago but it does not bother me and seems to be no problem.

I use my mac for gaming sometimes, I would like to play HOTS with my girlfriend but it's not capable of it right now, though I can run League of Legends without issue (apart from very slow loading times into game, once in i have no problem). I'm aware I cannot replace the main graphics card, and I do not want to use an external one.

Very soon I will be starting to DJ live with my mac using Traktor, it's not very intensive, but at the same time if I do happen to run into a problem live it would leave me in a very embarassing situation with a quiet dance floor, not what anyone wants haha. It should be noted I have never had any problems with Traktor at home, it has never crashed, I have never had the spinning beach ball while practicing.

My plan would be to replace my HDD with a new 1TB SSD, and also to upgrade the RAM (either add another 4GB in the empty slot so I have 8GB total, or take out my 4GB and add two 8GB modules for 16GB total). And of course to change my battery.

The only thing stopping me is that I cannot change my processor, I have the 2.5 GHz Intel Core i5, and I know that the latest models now have the new i8 quad processors, I'm worried that with an old processor I will still have problems with new software despite the changes I plan to make.

Any opinions would greatly help me, especially from anyone who has upgraded my model of mac themselves. Will the performance boost be significant if I upgrade? Should I go for 8GB or 16GB of RAM? Will I be able to play HOTS? Will my mac run reliably with Traktor in a live setting?

Thank you for your time, it's greatly appreciated!