Note that I don't play grey knights but I've faced some very hard GK armies in 5th and not everything I say should be taken to heart but I'll try and help you out.
Notes about GK:
-having all infantry makes the army very slow
-many GK units can do a lot of damage to light infantry, but their own infantry are expensive
-all the most powerful units are not the normal troops
-the majority of non-troops choices can get more/cheaper psycannons per less models (purifiers+paladins are the most noteworthy)
-psycannons are very powerful and generally are the driving force of this army
-coteaz can be used to change the army completely, giving you inexpensive troops and vehicles (psybolt ammo razorbacks with heavy bolters were very common).
-psybolt ammo is powerful on large units.
-Grey Knights are very balanced, and many people consider them the best codex because they have few real flaws if you don't buy too many upgrades.
-always look for the less expensive route. Its normally better.
and maybe the most important note
-Grand Masters allow the GK's to have an easy time generating scoring units. While other armies may struggle to obtain powerful scoring units, Grand Masters can make many units count as scoring, which allows the GK's to do (I think) ridiculous things like having a scoring Dreadknight teleport/shunt onto an objective.
-purifiers, dreadknights, and dreadnoughts were the choice units to make scoring using the Grand Master's rule. Interceptors were used by some people, most notably Blackmoor, who apparently was one of the most successful "Draigowing" army users. He's quite a skilled player and you can find battle reports of his around the web.
on to the units you are considering:
Grand Master: a great unit, upgrades can make him very expensive. The incinerator seems like a nice upgrade. Draigo is great for being unable to be instant-killed, but the Grand Master already has Grand Strategy, the best aspect of any grand master.
-If you want Draigo, read up on how other people have used him. While Draigo is a very strong character, him+paladins actually aren't that easy to use because of how slow they can be.
-In the last grand tournament I attended, the two top players had Draigo in their lists. One had Paladins, the other did not. I lost to the guy who did not have paladins, and while I got unlucky (guns failing to kill any of his tanks in kill points) I saw that draigo is great just for sitting in close combat and staying alive.
Terminators: some people like these, but buying 20 of them is a big investment. I would use 10, and play with both using paladins, or regular terminators. See what you like, and then go from there.
-the weapons you chose seem perfectly fine, but you might want 2 psycannons in each squad so they can kill enemy tanks.
Purifiers: you probably want to give them 2-4 psycannons. If you didn't notice, they get rather inexpensive psycannons, and get more of them per model than PAGKs do. Also GK's really do need a lot of shooting power to do well, or else their units get shot down before they get close enough to do a lot of damage.
-PAGK's are less expensive. If you know you can keep purifiers alive, they can do more damage, but if your opponent targets them, you will realize they aren't so resilient. Many people used to buy transports for purifiers in 5th.
-most people buy all halberds for close combat, and a couple hammers to help with tanks.
Dreadnought: the twin-linked autocannon with psybolts is one of the best anti-tank units in 40k. Avoid guns like lascannons, that are more expensive, and actually not as powerful as the improved autocannon. This saves a nice 20pts over the lascannon.
So yeah, that's my pile of writing. Hopefully that helps.
