SWMBO and I have our permit to stay for over 3 months in Bulgaria.
I would suggest you take an English speaking Bulgarian with you to show you the ropes.
I cannot advise about Plovdiv, as our dealings were in Dobrich at the main police station initially.
Documents to take : Passport, European Health Card, I took my Local Municipality Tax Bill,(to
save translating my address).
At the police station you will be given a form and have a photograph taken.
We then went to 3 different Notaries to collect more forms, one of which is not to become a burden on Bulgarian Social System.
It will be advantageous, if you can get your passport(s) and EHIC card(s) to expire at the same date,
else you may end up having to go there about once every 3 years, according to which of these expires first.
Next day the permit is ready.
There have been posts previously, so I stand to be corrected.