i'd refer you to this thread, since its pretty much the same question answered, and there's more info linked in it as well.
Short answer, SQL express is probably enough for 500. The main stresses on LANDesk appear to be on the core itself. How many procs you can throw at it, and disk speed for the file writes that come in a lot from machines depending on how their agents are configured, ie inventory scan files, security scan files, blah blah.. There's a lot of disk access because of this, so be careful on things like tryin to make 500 computers do something all at once!
SQL express vs sql server does not affect performance.
Generally SQL server may give more performance as it is install on a separate server and SQL express is generally install on the core server which consume some ressources.
SQL express as limitation compare to server like maintenance plan wizard or DB size.
Important Note: Microsoft SQL Express editions are severly limited in CPU usage, memory usage and total database size. This can cause performance problems and even complete failure if those limits are exceeded. It should only be used in very small environments or test labs.
Please check this article for architecture requirement based on the amount of managed devices with LDMS: