Just looked at T4 sevvy list, and its pretty damn cool
From what you have and want to do, i have a made a list going from 15->35pts of how to build it up into T4 etc. and hopefully keep prices equal between point levels :O
15points
Grand Scrutator Severius -6
Repenter 4
Blessing of Vengeance 7
Choir of Menoth (Leader and 3) 2
Temple Flameguard (Leader and 5) 4
Temple Flameguard Officer and Standard Bearer2
Hierophant 2
The repenter is a Cheap jack points and money wise i think its our cheapest jack. It has a flamethrower which if anyone survives, sets them on fire

. The choir is there to help buff up the repenter aswell as the Blessing.
the repenter combined with choir buffs and pSevvy Eye of menoth. It is a DESTROYER OF WORLDZ.
The flameguard and
UA will help start off your infantry base. the
UA will give all the flameguard Auto-matic fire on their weapons.
Just what you like

I think its a pretty powerful list and still fits in his tier list, but you just can't benefit from it yet.
To make the list 15pts Add the guardian and 3 wracks
I like the guardian with Sevvy because he is a heavy jack you need to destroy enemies with. And he also has an additional arc node on an Already tough model to deal with. so now you have 2 pretty hard to remove arc nodes and some Heavy backup!
The wracks are just to fill up that spare point and are really nice with Sevvy because having more focus is Fun!
Now to make that 25pt to 35pt might be the most expensive but will fill all the criteria you want.
You will need to bring up the original flameguard unit to 10 models by the addition of 4 more. Then buy another 10 Temple flameguard and another
UA. sevvy tier list gives you a FREE
UA!
Now you have two massive units of flameguard who all cause automatic fire. Combined with Eye of menoth. That is Sickeningly awesome
Also 2 vassal mechaniks.
This brings you to a T4 list at 35pts with LOTS of FIRE and FLAMEGUARD! 2 small jacks and 1 big jack

and 2 mechanics to follow your arc nodes around
Making any army is expensive, but i think this is probably the best way to go about building it.
comes to £138 on maelstrom games :O
but over the course of months and such buying and painting 1 unit at a time.
getting to 15pts is around £55, 15>25 around £27, and 25>35 around £55 , could probably change the stuff around to even it out a bit more.
You can make cheaper armies but they wont be as awesome as this
Hope it helps!
Had i the money i would be making those lists asap XD unfortunatly i went with Kreoss and knights, but it suits me better then flameguard