I don't think Master Spellthief does a single thing without multiclassing, so don't bother if you're going single class.
What it does for single-class spelltheives is that it makes your caster level equal to your level instead of 1/2 your level (as it is normally), which isn't that much compared to what it does for multi-class spelltheives, which I why I'm putting in as a "maybe" option.
Honestly I don't understand what the big deal is about this feat and multi-class spellthieves. I've seen all kinds of arcane caster level abuse with this feat, but I honestly don't seeing it do much for spell-thief focused characters.
Master Spellthief increases your spellthief level
only for determining
the highest level spell you can steal. This doesn't include
any other abilites or aspects of spell-stealing. I've read the wording on both the feat and the class and also read some threads on master spellthief and from what I can tell Master Spellthief
doesn't increase the maximum amount of spell levels you can hold, which is equal to your class level.
I see builds like this (from the
Master Spellthief thread):
... the following might be better suited for it:
spellthief 1/wizard 5/spellwarp sniper 5
that is, only if you're considering master spellthief, which i'm fairly certain you are. that'd give you steal spell CL 6 and a wizard CL of 11...
That's nice and all, but being able to steal 3rd level spells isn't really helpful when you can only hold other spell levels. I've seen other builds like this that use master spellthief to pump their steal spell ability to 9th level, and yet they only have a few levels in spellthief, so they can really only hold low-level spells. I'm not really sure if they're aware of this or if they didn't notice that the feat doesn't boost that aspect of spell stealing.
What's Improved Crossbow Sniper?
I found it
here I'm not sure what book it's in (if it even is in a book).
That said, a Spellthief/some primary arcanist class/Unseen Seer with Master Spellthief would probably be stronger for that than just Spellthief/Unseen Seer.
You're probably right, but our group had a really high-powered campaign last time, so we're trying to keep it low tier. Basically I'm not
too concerned about sucking a lot. I'm open to multiclassing arcanists, but I'd rather not sacrifice a lot of spellthief class features (which I think are really cool).
EDIT: I take it from the 4 starting feats that you're a human with 2 flaws? I guess you need the feat, and it's a darn shame the rogue race I most like has a charisma penalty. But I do rather like Whisper Gnomes (Races of Stone) even so. Small size bonuses to attack, AC and Hide; dex and con bonuses (str and cha penalties); speed 30 despite being small, low-light AND darkvision; huge racial +4 to hide and move silently (if you're keeping count, size + dex bonus + racial = effective +9 to hide), spot and listen bonuses, some useful spell-like abilities...
Since the charisma penalty is probably a non-starter for you and you do need all the feats you can get, look into the Strongheart Halfling if you can use campaign setting books. I think it's in Player's Guide to Faerun, but I could be wrong, I've never actually seen the entry myself, just heard about it online. Basically they're halflings (small size is very nice for a rogue, again) that trade some of the normal racial benefits for the human bonus feat.
Actually I was already planning on going strongheart halfling. The Whisper Gnome idea looks cool but I think I'll pass.
EDIT: Oh I forgot to mention the other party members. One is a Half-Minotaur Swordsage using a spiked chain, and the other is a master of many forms.