Not a big fan of the Impostor Background, especially with Arbites, I reccomend to look at the others, especially the IH ones (though the RH, Justice before Law is an interesting one if your methods are more vigilante)
On gear, 1/2 price is weak, especially for higher ranking players, why would you have the chump gear that a new arbites would get, you are getting a free cyber mastiff worth about 3k+ (for IH values), the BoJ ones are even more expensive. I can understand the GM not wanting super cheese, but there are limits….
Shotgun and 12 shells Sell, keep shells, alternate idea, turn it into a hackshotgun if this is a double barrel, see if GM is willing for you to buy a proper shotgun instead (at least pumpaction) if you sell at full value, otherwise 30 thrones… (Side Note, the Hack Shotgun is pretty darn good, especially if the GM lets you be able to fire it one round at a time as well as double mode, even without it, given quickdraw and maybe rapid reload it is a nice emergency gun and looks cheap too
Club-if you want a cheap hard hitter before shock, BQ mono upgrade is 85 thrones, 1d10+1+SB Pen2 +10 to hit
Brass Knuckles-Keep, BQ or GQ if you like fighting
Mesh Vest - Could be good to keep, (150/2) = 75 thrones if sold, which is a ripoff
Stimm + Injector, this stuff can save your life, not often though, but it can be amazing
Arbites ID-useful for dealing with enforcers
Lhos/Amasec-Keep, how else can you fulfill some tropes?
If only 400 (and if you are doing an infiltrator) DO NOT get the Arbites special shotgun (either the HUEG round one or the awesome Persecutor), remember NPCs by a good GM are not dumb or blind, there are many shotgun options. The cheapest (and cheeziest) is the Ironclaw for 80 thrones, add a puritan for 100, stick with the mesh vest, get cheap armor to supplement, get whatever melee upgrades and weapon upgrades you can afford and some good gear
The 40k armory is pretty good for having ALL the things from ALL the books, for a real nice collection with some great fixes and price changes, check out the Macharian Handbook, has two books so far, a armor and weapons book and one with GEAR, see if your GM may want to use that, it really helps having almost all the items in one book (minus the heretical/xenos stuff overall, that will probably be another book), it is fan made, but VERY professional, though in the process of being updated for new books. Slugs and some other cool stuff (Bolts, Flechettes, Gas Rounds and Stun Shells for shotguns) comes from RT Hostile Acquisitions. It also has rules for making primitive armor lose the primitive status, assassin's longlas, shotlasguns, an arbites model awesome combat shotgun, making almost any armor into "powered" armor and those are just some of the GEAR options, it is a pretty fun book, like BoJ in my opinion, not something for everyone like IH/ItS, but if you have the cash, worth grabbing.