Had mold remediation done in my dad's house in the master bedroom and tiny bathroom. Was not the terror-inducing stachybotrys "black mold" that everyone freaks out about, but it was considered a dangerous mold for anyone with a compromised immune system. House was a hoarder house, and had sat packed to the gills with crap for over 20 years and the mold was from a leaking window and broken toilet (two different areas effected).
It cost us right around $3K for all of the cleanup and testing (before and after to make sure it was clear). This involved them removing the drywall, interior insulation and some of the flooring where the mold was in both rooms (just a 8 foot section of the bedroom, but the half bath was down to the studs demoed) and disposing of all, wiping down every surface with a mold killer and then negative pressure ventilation to remove the mold throughout the house. This was something that technically we could have done ourselves, but I lived out of state and we were planning on selling the house, so we went with a professional remediation so there were no questions that it was done correctly.
What the mold tester guy told me was just because it looks scary, doesn't always mean it is scary - and just because it's black doesn't mean it is the dreaded black mold. There's a whole bunch of garden variety mold that is black, and the guy said that there is a ton of outside/inside mold spores in every house, but folks don't generally realize it because for the most part they are benign. You can't know what you've got unless you test for it... the testing was about $200 a pop, but it really helped to see what we were dealing with (and we were pretty sure it was bad since the leaking/moisture issues had been going on for YEARS). Oh - and the mold tester guy was not affiliated with the remediation company in any way. We got him from a realtor referral so we knew there was no conflict of interest.
If you feel comfortable doing it yourself, I would probably go that route since it would save you a good amount of money. Big thing is just demoing the mold stuff - like ceiling tiles and drywall/insulation that got wet - and anybody could do that, and then hanging drywall or redoing a ceiling... But you'll probably want to get the house professionally tested after removing all the soiled stuff and wiping everything down with a mold killer so you can make sure that it is mold free and not lose any more renters because of it. Before testing, give it a week or so for the dust to settle and air quality to improve - but BEFORE closing up the walls/ceilings and might want to air it out really well or rent some big fans for the first day or two to get the air vented outside and help move any residual stuff out. Definitely do some research on the DIY stuff before getting started!