The Great Bear King said:
The necromancer will need to study the corpses of the dead to learn the necessary surgical skills to become a physician.
That is true. I know that even Leonardo da Vinci needed dead bodies to perform his anatomical studies. After death he had only IIRC two days before the dead bodies started to begin foul smelling. Leonardo did not open graves, he paid and bribed persons to bring him bodies.
The Great Bear King said:
He's gathering the bodies himself (or herself if you prefer) to get around the local thieves guilds (who tend to be hit men, thugs, and particularly murderous brigands).
Most necromancers want knowledge to become a physician but they certainly do not want to waste their time by digging. To save their time they pay some unscrupulous persons who perhaps could be members of the local thieves guild.
Their saved time they can invest in their studies. If they do not dig themselves, they do not need any rogue skills. Learning any skill costs time. Advancing as a rogue instead as a necromancers costs also time. Why should a necromancer shorten the time he can learn necromancy or related knowledge by learning skills he seldoms need.
A rogue/necromancer never can be better in rogue skills than an entire thieves guild which is his rival. Perhaps the rogue/necromancer can fool some low level rogues, but if the thieves guild know that somebody steals dead bodies they will set a trap for the necromancer.
To learn rogue skills he needs a trainer. If the trainer was from the local thieves guild the tieves guild knows he is a rogue.
A necromancer can cast more powerful spells than a rogue/necromancer of the same CR.
If every necromancer needs dead bodies the thieves guild know this fact and selling dead bodies to necromancers is a way to gain profits.
I am not saying that your character concept is bad, but your NPC is an outsider even under his fellow necromancers. He is an outsider because he has not the money to pay for the dead bodies. He has to dig himself which limits his time to study the bodies and to study necromancy and he has perhaps an enemy in the local thieves guild.
The Great Bear King said:
Think of the 16th century, where grave robbery is a misdemeanor (as is killing the grave robber) and theft of goods is punishable by hanging.
IIRC A dissection of a dead body was forbidden by the church.
The Great Bear King said:
For your special grave digging necromancer I suggest a human rogue 2-3/necromancer 1-2
or a human necromancer 2 with the able learner feat and some skill points spent in rogue skills. He has no masterwork weapons because if he had money he could buy the dead bodies from the local thieves guild.
The Great Bear King said:
All with racial CRs below 6.
All intelligent humanoids.
I can think about a drow rog2/necromancer2 who works in a city at night only.
I can not think about a reason why this drow should be a good-aligned drow but this is your problem

He lives in an abandoned haunted house (he used ghost sound and prestigitation to create the rumor or a haunted house). Nobody trust a drow so he uses the alter self spell, but he can not speak common very well so he has not so many contacts in the city.
Or a half-orc or other monstrous race like hobgoblin, bugbear or goblin rogue2/necromancer1 who was a former thieves guild member but he found somewhere a magical book which teachs him necromancy. He abandoned his old ways but he knows nobody like to deal with an ugly half-orc or other monstrous races.
The Great Bear King said:
levels is the plural of level. The minimum for the use of the word "levels" is 2(levels).
2+ means at least 2 level, but it can be more levels.
The Great Bear King said:
Not always, but the majority will be.
If you want to understand why the necromancer's master doesn't work with the local thieves’ guilds, please read this.
Do you suggest that in your campaign grave digging and stealing dead bodies is legal?
If it is legal you need unscrupulous man to do the work.

A normal fantasy world is not 18th century. The time of a fantasy world IIRC is medieval.