Well be warned that the codex is dated and can easily be considered the weakest in all of 40K. That said, you should also go for the army you think looks the coolest. You seem to really like the Daemon Hunters as a whole, so I would advise you to start them.-Is it a bad idea to start off with a daemonhunter army?
Even though they aren't easy to win with, it's still possible. If you don't mind taking advantage of the Allies rule, you can even had some great elements from the SM and IG codices. DH armies generally lack numbers and anti-tank weapons, so taking a couple squads of IG can really help.
Lots, sadly all the infantry is metal (unless you use allies) and the wonderfull GK terminators are especially expensive. Also keep in mind that the 5 man Grey Knight box is pretty bad value for money as if you buy many you will end up with way too much Justicars and Incinerators.-How much should I expect to spend?
Again, I would advise that you go for what you think would look the coolest. As an exemple, pure Grey Knight armies are really hard to win with (some may disagree, but I would dare to say impossible to win with against certain armies). If it can help, HQ wise I would strongly advise a GK grandmaster. He his a monster period! He's competitor, the inquisitor lord isn't, at most he can make an ok heavy weapon squad, but... he allows you to take some great units such as assassins.-What would your suggestions be for a good beginning, basic, and balanced army, using grey knights and centered around majority inquisitorial forces?
Storm Troopers aren't the greatest troop choice in the world either, so I wouldn't load up on them too much. I wouldn't be ashamed of min-maxing my squads to carry as much plasma death as possible. A mix of GKs and Guard infantry could be a good compromise. Termies fortunatly are as good on the table as they look in the display cabinet, by all mean get some.
Depends who you play. By WYSIWYG that wouldn't be permited, but if you at least try to bulk up the Cadians I'd consider it ok. While the DA vets would look cool, they would cause problem because they are a unit that exists in another codex and an Inquisitor retinue is far from having the powerlevel of marines.-Is it okay to take some liberties on model choices? For example, I really like the cadian kaskrin models to represent the storm troopers, but I if I were to have two units of them, it would cost me less then half the price and still look fierce to use cadian shock troops all jazzed up in red and grey. Also - if I were to order the Solomon Lok forgeworld figure for use as an iquisitor, could I use some dark angel vet marines for the warriors in his retinue?
If you want to deepstrike GKs, you could mount Storm Troopers with teleport homers in chimeras and hope for the best. I'd like to give you better advice but I haven't played with my DH for a long long time. I have been playing with pure GKs a bit though so if you're interrested in that I would be of much better help.-What kind of stratagy would a solid army be built around? I like the idea of a lot of deep striking GKs but I really don't how good of an idea that is...
Welcome to the Ordo and remember not to give up if/when you lose your first couple of battles.