As a licensed Premium user, do I need to upgrade to the Enterprise now or can I do that later depending on the requirements of the questions above?
You can do this later.
Version 4 has an integer primary key on the tables compared to the old varchar key this is much faster.
For 30000 computers the dashboard can be slow depending on the types of reports you need on the dashboard. Your database guru can tweak the indexes needed to speed up the reports you make.
If you need the report only occasionally, leave it off the dashboard.
Lansweeper service requirements:
Memory: not much, 100-200 MB max depending on the amount of threads you run.
multiple cores/processors allows you to use more threads in a faster way.
In your situation I would recommend a dedicated scanning server (or multiple, this can be tweaked later)
Database requirements:
For everything: more is better.
Memory,CPU,Diskspeed.
To speed up everything I would enable only the items needed (some items are already disabled by default, dcomapplications,proxy,...)