Need some help trying to gauge the difficulty of changes to a monster my PCs' will be tackling soon/ encounter.  The backstory is they found out a noble that they were trying to prove was in blackmailing/extorting people was an all around bad guy, surprise he was but was also a raksasha in disguise of the noble they were investigating/tracking( raksasha killed the "original" and took his place. He has framed the PCs' for varies murders/ foul deeds and they have gotten to the point where they can take him out/ need to. The problem I am running into is how the encounter will stack up to the PCs'

1. The raksasha is more a caster/ stay out of the fight type of villain. So i took the base creature and took away his innate spell casting and made him a full sorcerer up to casting 7th level spells. He still has all the other abilities that a raksasha has I just moved his innate spell casting in to spell slots. I just can't gauge if this would make him more powerful or same( my guess is more but I am not sure).

2. Can't think of how many body guards/ helpers or the type he would have with him to help when the PCs' show up.

I am trying to aim for  hard to deadly fight as the PCs' are well informed of what kind of person/creature they are dealing with, they know this villain will have help. The PCs' are all 10-13 level the back down is 

Dragonborn Paladin lvl 13 (oath of Ancients), Tiefling Fighter(3)/ Lore Bard(10), Gnome Divine Soul Sorcerer(12), Human Ranger Hunter(13), Elf rogue Scout(11) and helping them they got some allies to tag along Human Gunslinger(7)/Sorcerer Wild Magic(4), and finally a Human Wizard Abjurer(12) 

All the characters a some from of good so they have that going for them but I just need to know how to set up this fight or how it would go.

Any help would be great from those more experienced would go a long way

So, what I think make's it hard to give you exact advice is determining what the CR of your new Rakshasa is. Their base CR is 13, and with it's move set and abilities, unless all the party doesn't have magic weapons (but even then) this would be an extremely easy fight with this creature alone. With your homebrew additions, not knowing exactly what other spells you gave, or features, I would say at the minimum you've maybe increased the CR by a few points? Again, I'd have to know what spells you gave. Traditionally, the Rakshasa spell list is not offensive, so alone anyways it has a hard time to pose a  challenge to the players in a "typical combat" setting; obviously based on it's build up, I'm sure its non-offensive spells have come into play making it a powerful roleplaying opponent. So, I'd really recommend doing what you can to look around the internet to help determine an estimated CR for your new Rakshasa so then you can use sites like "Kobold Fight Club" to determine the difficulty of the encounter and what you need to make it go from an easy to medium to hard to deadly encounter.

At the very least, I'd toss in a few lower level CR fiends that have aided the Rakshasa as goons, maybe they were also disguised one way or another. I bet 2-3 disguised chain devils and your rakshasa would be a pretty fair/challenging fight. I think since the Rakshasa is a fiend himself, it'd make sense for him to have some other "weaker" fiends surrounding him that he was able to control.

If you provide more information or have some more specific questions, I'd be happen to help further.  

Rakshasas are fiends, so you could have them summon help from Infernal sources, or maybe he can have a simulacrum of himself, like the simulacrum spell, but he can have multiple. Apart from that, I don't have many more suggestions. I hope you can use this!