Hi,
I was DJ-ing with vinyl in the 70s and am also studying the digital music scene now. To test my vocation, I decided on Serato as my main DJ software, and used eBay to get a secondhand Vestax VCI-300. This comes with Serato Itch, but Serato DJ is a free download for the full version. I’ve also ordered an Akai-AMX because maximum portability is a goal (my nearest city is a long bus or short plane-ride away).
The VCI-300 has gain knobs for its two channels, although they are marked “Trim”. For other controllers, I would start with the Serato site’s Hardware page and check out those controllers listed as being “Serato DJ Enabled” as opposed to “Serato DJ Upgrade Ready” – the former is a free download for the full version, the latter a paid upgrade.
Hope this helps.