I am planning on buying my first few miniatures to play X-wing after a friend introduced me to the game. After playing a few games with his miniatures I most enjoyed playing scum and Kavil in particular. I would like to play that faction but it seems inconvenient since you can't get scum ships from the starter set which seems like a waste of money and large ships seem more expensive for their points.

so my question.

A. How much more expensive would playing scum really be?

B. How viable is Kavil after I quit being a noob and what models go well with him?

It cost a bit more to play scum I guess about 10-30$ more depending whether you use the ships in the box or not. If I were to start playing scum, the first thing to get is a core set. I would advice to buy the Force Awakens core set because if you want to play tournements at some point, then you will want the new damagedeck and you do not get that in the old core set.
Reguarding scum expansions I would get the Most Wanted expansion, because it has 3 ships and so many things you will want (incl. Kavil). I would also get the Scurrg H-6 Bomber expansion, because its a great ship and you get some great upgrades. Lastly I would get either the Punishing One Expansion, Protectorate Starfighter Expansion or Slave I Expansion depending on which ship you like best. With a core set and 3 scum expansions you can make some good scum builds and I am pretty sure, that you will not regret buying any of these expansions.
It will cost you between 115$ and 130$ for this, but with this you get a good really good base for playing scum, that I am pretty sure you will never regret buying - you get really good ships and upgrades.
Force Awakens Core set 40$
Most Wanted expansion 40$
Scurrg H-6 Bomber expansion 20$
Punishing One Expansion (30$), Protectorate Starfighter Expansion (15$) or Slave I Expansion (30$)

One of the most useful tools that a new player can use when pricing a list is Han Shopped First . It takes a standard text output (called xws) found on most of the online squad builder sites. It then outputs a list that shows you what expansion packs the ships and cards come in. Even better, it suggests one or more ways that you can buy the list the cheapest.

Fab's Squadron Generator also can tell you what expansions you need for a certain list by building a list then clicking on the "Math" button at the top of the list area. It also has prices, once you select a currency to display the prices in.

Playing scum, and in particular starting with scum can be very expensive if you are wanting to go to tournaments because almost all the cards you need (VI,PushTheLimiti,TLT,Expertise,etc) are in other factions.

If your not playing in tournaments then just proxy away and just get the ships you want to fly. Most wanted for kavil, scurrg for Nym, jumpmaster cuz its the jumpaster.

Thats like $90 + core. Not too bad.