Welcome aboard TFR!
Good solid bikes, those 125Ds. Make sure it's in good condition, and go and ride it.
Far too many people delay their entry into VMX participation because they feel the need to build some world beater bike...
I should have started racing 15 years earlier than I did - the bike I was turning into a world beater is stashed in the back of the shed, in a million bits... Meanwhile the two bikes I just bought and rode (including my 125D) have both been to plenty of events.
Back to your 125D:
There's a million possible upgrades. Don't worry about them, and ignore anyone who suggests that you "need" them, particularly if this is your first foray into racing.
When you race it, you'll quickly form a list of things you want to improve/change.
I'd suggest a thorough look at the bike, and make sure it's mechanically sound.
A basic rebuild of the shock absorber and a change of fork oil is a good move - one that's often overlooked.
Make sure the footpegs are tight on the frame. They eventually strip the splines if run loose.
Good luck, have fun, and see you out there.