Well, if your Dm is crazy you could use the DM toolbox rules to make a monstrosity with Hit Dice and levels!
The only thing: According to the DMG, monsters that gain the same hit die for class as their monster hit die which I do not like. For instance, a Pureblood Wizard 1 would get 10d8 + con*HD (monster hit die are based on size according to the book) while I would house rule they use the class hit die resulting in 9d8 + 1d6 +con*HD.
They could make decent rouges, warlocks, or ranged fighters.
Animal Friendship (snakes only) is nice flavor.
Poison Spray 3/day gives some elemental damage on top of class abilities.
Suggestion 3/day makes for interesting RP and combat scenarios.
Magic resistance is always nice.
While CR is a bit wonky, CR 1 means something like that could probably travel with 4th to 6th level party and not be too much to deal with for a DM. Yeah, Hit Dice. But you would not have the class features and spell levels of a pure level 4 to 6 PHB character.