Highlines are 2.8 with no extra power.
There are a few differences, mainly trim.
Black interior A B and C pillars, Plastic sill cover with VR6 on it, slightly different centre console and rear ash tray, VR6 badges with silver writing on a black background front and back. All very pedantic really and not worth extra money.
Colour coded heated leather and Manual A/C is standard, climatronic was an option as with any MK3.
Highlines have 15" 6 spoke BBS solitude wheels the same as colour concepts and coraddo storms.
No real reason why a Highline should be worth more than a Normal VR6 with full spec, unless you desperately want purple Violet pearl metalic as a colour, which was exclusive to highlines.
I don't think highlines go for a couple of grand more than a normal VR6 but maybe a few hundred.
I say go for the best condition VR6 you can find.