Point 1: Add one more condition to the WHERE clause.
WHERE
tblAssets.IPAddress LIKE '192.168.1.%'
AND tblAssetCustom.State = 1 -- active
AND tblComputersystem.Domainrole > 1 -- server
AND tblDiskdrives.DriveType = 3 -- hard drive
% is a wildcard, so you're saying IPAddress starts with '192.168.0.'
In case you weren't intending to limit your query to servers, see tblDomainRoles for the role names.
As to the second point, just divide by another 1024.
Base units: bytes
/1024: kibibytes
/1024: mebibytes
/1024: gibibytes