Welcome to BGW !!!
I assume this IS a controller cart? IF SO, then yes you will need to purchase a 48 volt controller. The controller will have to be matched to your speed controller. So you need to know which type system you have. I'm not familiar with yours but it should be 5k-0 or 0-5k? Buy the biggest controller you can afford; amp-wise. You will get a wiring diagram with your new controller.
Are you going to go with six 8v batteries? That's the cleanest install, IMO.
There are many different chargers available. No
one has become the go to model. However there is something to be said for the manufacturers that have been around for some time which is why I use Lester chargers.
I like one that puts out a max charge of between 21 and 25 amps, and has 3 stage charging, meaning it will excite them, then charge them, then finish them. 3 cycles at three different voltages. Many brands do this in a fully automatic charger. You just have to find one that fits your budget. You will like all the benefits of 48v.
Good Luck