You haven't built the garage yet? do a standing seam steel roof and you will never have to replace the roof and you will save $1,000's on the solar install, a big chunk of that cost in the quote you got was most likely installing the roof mounts, out of the 3 day install I would guess that 2 of those days would be spent cutting holds in your shingles, installing brackets and installing expensive flashing boots, on a standing seam metal roof you don't cut in to the roof, instead the panels attach right to the standing seams and clamp on, saving the installer days, preventing them from buying expensive flashing boots and it saves on panel racking because the roof becomes the racking.
To answer your original question, I don't know of any situations where a single large inverter makes sense any more, other then in off grid or battery back up houses, personally I'd go with something like
Helios Solar Works Solar Module Description and Specifications and use their A/C panels that have the micro inverters mounted right to the panel, I can't find the price right now on them, but when I looked before they were around $2 per watt! They are also made in Wisconsin, not China.
Around here the panels do not need to be installed by a "solar installer" to get grant money, but the grant money is based off of estimated output and that output is figured based off of location and shading, so you'd first have to have a site assessment done and they would give you a very accurate output figure.
At least here, if you own the house and live in it you can do electrical work, you still have to get it inspected by the building inspector, the other option is to find an electrition that is willing to work with you, they are not likely to want to let you do much of the electrical work, but they will most likely let you do a large chunk of the non wiring labor.
So talk to someone in your area that does site assessments and they should be able to tell you more about what you need in your area.