If you've never had a dog as an adult, a dog-aggressive one is not the place to start.
Absolutely agree with this. And I know because I've done it.
Even now, four other dogs and a lot of learning down the line, I wouldn't knowingly take on a DA dog. Even when it's mild it's very limiting. If it's severe it's a complete nightmare.
We had DDog1 for eleven years. For five of those years we tried, with the help of numerous behaviourists and trainers, to "fix" him. After that we realised that he was never going to improve and it was going to be a case of managing him. In his case (his aggression was fear based and very severe) that meant no longer walking him as it was just too much for him to cope with. We were lucky in that he adjusted to life like that very well, had he not our only option then would have been PTS.
He was very, very happy as a house dog but that left us with a dog that couldn't be kennelled, or home boarded. We had the odd self catering holiday but cottages had to be chosen by their proximity to other properties in case there were other dogs nearby.
Trips to the vet (of which there were many as he developed a heart condition and arthritis in later life) were like military operations.
He too was fabulous with people, he really was, and we loved him dearly but it was always disappointing that we couldn't take him out anywhere. Always a strain knowing that we couldn't do this, that or the other because literally everything had to fit round him.
He was an extreme case but it's not difficult for an already DA dog to get worse.
From experience, taking on a dog that is known to be DA as your first dog with no back up or support is utter madness.