I don't have much advice other than to take time to actually try out some hardware before you commit to buying it. Ask your local DJ store if they can set up some or all the ones you're considering for you to come in and test.
The industry doesn't make it easy for (some of) us in the sense it can be difficult to find one piece of equipment that has everything you want, but there is a wide range of choices now so at least there's that. But with so much kit to choose from (some of it fairly expensive), serving different scenarios and needs, that it's worth taking the extra time to be sure you're getting the best you can for what you like and want.
And although you like the 505, since you say you're coming from Reloop/DJAY, it's unlikely to ever be supported (
community.algoriddim.com) so you might want to consider a hybrid unit that works with both platforms? There are many that are compatible with both systems.
www.algoriddim.com
Another thing you'll need to consider is the computer you intend running Serato on. It may need to be quite powerful and (e.g. in the case of the Mixon 8 Pro), it may only work with a more recent version of SDJ which may require a more up to date computer / OS. So be sure to check specs and system requirements as part of your research.
Other than that, I just wanted to say welcome to the Serato family. :)
DJAY's dope but so is Serato. So you now arguably have the best of both worlds.
Best of luck.
