Hi, I have a similar scooter to yourself and I also had no idea when joined this forum. Its fair to say I still have limited knowledge but I know enough to help offer some quick fixes for you.
In short if you want a cheap and reliable battery and charger I would suggest you pick up 4 x 4s multistar 16000mah lipo packs (I got mine from ebay for £45 each) . This will give you 16s and 66-65v at full charge this might be a little much for your controller so as mentioned a 15 s pack made of 3 x5s packs will give you a full charge of no more than 63v and i think your controller might be ok?!? but finding cheap 5s lipo packs with hi capacity might be a challenge.
You will need to do a little wiring to link these packs in series and this will be able to be linked straight up to your stock controller. For charging the cheapest option I have found is to get a Lipo rc charger and pending the wattage you can charge the all 4 packs in series in one go.
I did this for my chaos 48v scooter (12s 16000mah lipos) and I can assure you your range will be massively improved! SLA’s belong in the bin! They will get you moving and if you only use the scooter from time to time then sure they will work out for you but with lipos you can increase the number of packs if you choose a better controller and motor so they will make your scooter future proof. if you went for 15s or 16s you would be over volting the motor a little and it would be ok with that but the strain would come to your controller (IMO)
As for performance you will maybe see a consistent power delivery and slight more speed and stronger acceleration ( pending your gearing) as the lipos wont sag like SLA’s do and you will be running for much much long.
Hope this helps. Have a look at some of the thread including mine, one of electric gods and ian hills as there is so much info out there! many of these scooters have been fettled with.
When you feel a little more up to speed with it all here is an amazing link that will walk you through building a good ev scooter. You will need a few hours to read and digest it so be warned.
Good luck and enjoy.