Whatever you do, I would say buy a wired in system

Batteries will just be a pain in the arse. And in a garage wiring should be easy, especially for only a couple of detectors.

I would also get a smoke/heat detector too ( heat if you will be doing grinding or similar work, as smoke's get set off too easily )
From a wiring point of view they're no different than a PIR anyway

Only awkward bit is most GSM type alarms seem to use battery powered detectors etc. Handy for install...but will require maintenance

A regular style wired house alarm will typically include a landline dialler, a GSM based dialer is usually an extra device.

No prices, but something like this

http://www.hkvstar.com/item/gsm-hard...em-gsm800.html

Oddly this seems to tick most boxes, and is cheap. Being sold off though.

http://www.ultrasecuredirect.com/aca...ess_Alarm.html