Depending on your preference I'd build a hack bike or get a cheap-ish road or CX bike with rack mounts and mudguard eyelets.
An MTB hack with slick tyres should be very reliable (7 speed drive train and Schwalbe Marathon tyres aren't light or bling, but they work so reliably comapred to many modern MTB components). If it looks appropriately shoddy you'll be able to lock it up anywhere, flat bars feel safer in traffic than drops/hoods and when you jump back on your MTB you'll appreciate how nice it is.
A road/touring bike will be faster and you might find yourself doing proper road riding in time, but it'll require more maintenance, brakes and drops mean it could be less confidence-inspiring to ride in traffic, and it will probably be more enticing to thieves, thanks to bikes becoming fashionable again.
I'd go with gears just for the versatility. It's an everyday mode of transport, not some test of manhood.
Whatever you do get a decent D-lock, not a poxy cable, and enjoy the fitness and the freedom you get from after-work detours and lunchtime jaunts.