Is there currently a feature that allows a exclusive checkout in SVN?
Is there a good plugin for Delphi that allows the access via IDE?
SVN has a concept of "locking" which roughly corresponds to the exclusive checkout. For example, in TortoiseSVN this is exposed via Get lock and Release lock menu entries.
JCL contains a SVN version control expert which works quite fine. Besides other things, tt gives you access to the locking functionality from the IDE.