Hacker’s Memory is more or less an expansion pack of Cyber Sleuth. So yeah, 2 games for the price of 1. I’ve put about 100 hours between the both of them, so there’s definitely plenty there if you want to get your money’s worth.
As for maining digimon, you can but with literally like 341 or so digimon to play with you’ll likely find some you’ll like more than others.
The Renamon line is easy to obtain and are some of the better ones, so yes you’ll be fine sticking with them if you really want to.