Your arena is looking pretty good, so I don't think you need to invest much in that.
Now your players seem to have problems. You need to get better players, especially guards. Excluding Primitivo Nivín, all the other guards have less than $3000 and obviously their performance is very limited. So you should get at least 1 guard with decent skills and preferrably a SF too (and at least you won't have to stick inside oriented tactics). You don't need to look for young players if you're not going to train them. Just look for players who are 27 or older (but preferrably under 30) so that you can get them cheaper.
As for you staff, $52 k looks pretty high. Look for advanced trainer, competent doctor and competent PR manager and your total staff salary could be as low as $30 k.
I'm sure other experts will also give you some more advice. Don't just listen to my advice but listen to theirs too. Follow the one you think will suit your team the best.