A warlock is actually great for a starting/just getting back into the groove player. I'm thinking about suggesting it to one of our new players. Dragonfire adept can also be nice, since a number of their invocations can be quite utilitarian and you still get breath weapon.
You say you're looking at buffing/debuffing, what about a dip into bard? The only reason I suggest it is Inspirational boost +Dragonfire Inspiration. Technically, you're not doing anything, you're just making everyone else better. Warlock gives you eldritch blast, or Dfire adept gives you a breath weapon, something you can rely on to do a decent amount of damage but not monopolize the table. Especially if you're using bard level dips for bonus energy damage via Dfire Inspiration.
Shame about the dragon magazines, there's a lot of fun in there, but if your DM is new or has had a bad table before I can understand. Dragon Magazines substantially open up options.
Spellwarp sniper is good, but ray spells mean ray deflection (spell compendium). Also effective with warlocks but not so much with adepts.
If arcane casters are already persecuted and your party has arcane casters, the thematic effects aren't going to be that big of a deal...hunted is hunted.
Transmuters can give decent buffs, make no mistake, but it sounds more like you're looking for a standard build that involves a low maintenance cost to ease you into things.
With the build you're looking at, you'll have to check to see if master spellthief will be applicable, since you're not technically a caster with only one level of spellthief (I know the boards count it, but your DM is using house rules, so I'd check first)
As for spell mastery...it's good if you know your DM will take your spellbook away. That's about it. If so, take a trawl through the boards for first level spells... s/he is allowing you to pick your first spells, yes? If not, it may be a waste.
Extend spell on it's own is good, don't forget sudden is once per day. Not terribly useful to a large party, but good if you're only buffing the BSF to be your meatshield. One level augment isn't too bad, and I'm assuming since arcane casters are reviled, you're going to have issues getting magic items (like a rod).
I do hope you have a crafter, because this could get hairy without one-maybe I'm reading in too much. It's four am, and I'm tired. And I have beer.. mmmm.... beer.
I do still heartily recommend Dragonfire adept for the low maintenance, though if you're willing to jump in with a wizzy, go for it, they can be a blast for versatility. Just be careful, because those spells you get per level may be all you're seeing. A dose of collegiate wiz wouldn't hurt (bringing it to wiz1/spellthief1/wix 4/spellwarp whatever) to increase those.
Domain wiz is also good as is grey elf. I'm not sure on your DM though, so I can't say much more...
