Hello there Snipper.
This lot in here are definitely opinionated, so I hope you get plenty of useful feedback.
This site has plenty of mapping resources and dev tutorials.
There are also various updates for the raw UT99 which will add newer features and capabilities. These may be useful for you.
To be blunt, one of the most useful tools you should get is original Unreal, then update it with the v227 patch from www.oldunreal.com
. That will give you more modern features in UEd2.1, including more import/export options.
You can prepare many resources in UEd2.1 for UEd2
With the 227 patch Unreal also has full static mesh support which UT99 does not.
A UT99 deco/mesh has cylinder collision, and U227 can use full mesh collision.