I did both, first bought a couple server racks to get my solar up and running,
then as I learned from others, and got my courage up, ordered 280Ah cells on Alibaba's (pay and pray) system and built my first DIY rack battery.
Let it run a few months and checked all cells were withing 3-millivolts so ordered more cells and built my second DIY rack battery. Repeat, cells stayed within a few millivolts of each other, so...
Now have my third set of cells on order, will be in by end of Feb.
I like to learn by doing, and take things one step at a time, and test/see if it works well. No problems so I built more. Not everyone is going to be comfortable to build their own battery, or be patient enough to wait for the cells. To each their own I say.
Cost-wise, the 280Ah cells cost about $1900 USD shipped, a BMS with active Balancer is about $70, through-wall battery terminals in 200A are $30, and a decent 2P 125A DC rated breaker for the battery is another $30, some wire and build your own case or set it up open on a shelf if you like. $750/100Ah approx cost. about half the cost of a factory finished rack battery in a nice case, perhaps more easily serviced. I like what I have learned doing my own. Like ET Cowboy, I like to build stuff becase I can more than becase of the money, the more I build the more I understand. If these last 10 years they did me well.