Totally new guy

I would start small (one or two heads) unless you know you'll have orders. One thing about running an embroidery machine in a garage is you'll have to be careful about dust and humidity. Any bit of dust in the thread path can stop the machine and humidity can cause thread to expand, thus causing the machine to stop. You would learn a lot about different machines and the embroidery business if you could attend the ISS show in Long Beach:

http://www.issshows.com/issshows/1237/index.jsp

Ted, I would suggest you grow your promotional products business to a success first.. when you can buy a singlehead or two-head machine out of it's profits with cash... then I would feel comfortable getting into another revenue stream. In the meantime sell embroidery, by all means, just subcontract it out to another shop until you are ready. Taking on too much at one time is my fear for you... and with 90% of new businesses not making their first anniversary in this country, I err to the side of caution. Slow and steady wins the race so to speak.
To help you with your new venture have you joined web sources like: distributortalk.com? they have nearly 8000 members that are mainly distributors but some are suppliers and others are service providers.... this would, IMO, be a very valuable resource to you so you are not re-inventing the wheel. Hope this helps.