It is confusing but perhaps your best resource is your local hood supplier.
When a range is given (like 600 - 1200 cfm) they mean that 600 cfm is the minimum required to draw steam, etc away effectively and under normal circumstances you don't need more than 1200 cfm. Generally more is better. Imagine it is Thanksgiving and you have all burners fired up going full blast. You will want a hood fan strong enough to manage it. On the other hand, you are home by yourself and cooking a little bacon for a BLT. 1200 cfm may be a bit over the top. The distance from the range top to the hood matters too. The further away it is, the stronger the fan needs to be. The manufacturer will usually give a maximum effective distance.
If I were you, I would go for a variable speed, mid to good quality fan with an easy to change/clean filter. As you might expect, the better the quality the more $$$. Going cheap is throwing money down the drain. You might as well not have a fan at all. I'm not sure safety is so much an issue with home hoods. In commercial kitchens it is as much to draw hot air away from the line as anything else. In the home it stops your kitchen from steaming up and it helps to prevent grease from settling around the stove area.
As to tying into the microwave vent, I dunno. It may not be rated for an exhaust hood fan. Again, ask you local supplier. Make sure you ask somebody with experience in these things; not some high school kid working a summer job.
Good luck and welcome to ChefTalk