More information is needed to give a meaningful answer. Budget in particular, spotting scopes range from £50-3000
Spotters are usually available in either straight or angled configurations.
Presumably this is for target shooting, so are you looking to spot the bullet holes or marker discs?
If you want to see the holes then you'll need to spend some serious money. 50x or greater mag and a large objective, the Kowa 883/4 is regarded as one of the highest resolution scope's available (around £2300 with a zoom eyepiece, you'll also need a very stable mount and head~ budget £200+ for both items). Look for ED glass, it gives better resolution, but again costs more, many scope makers produce the same scope in both ED/High definition and regular optical glass.
If budget is no object look at the new Swarovski ATX95 (up to 70x mag~ £3000).
Ace Cameras market the Avian line of optics. Good value for money.
Vortex Viper HD scopes are also well regarded, below £1000 they are probably the pick of the bunch.
Even with the very best optics atmospherics may make it impossible to see the holes at 600 yards.
If it's "just" the marker discs then it is a lot less demanding. 20-30x mag should suffice, which allows a smaller objective, you should be able to spot the holes at 200 yards and discs at 600 with a good quality scope and moderate magnification. Look for a secondhand Kowa TSN 613 for example. HTH