Are you planning to start selling them, installing or you want to use them for your house?
If you are thinking of business, I think you are in a good one, for sliding doors, i guess you are talking about aluminium stuff, you need to learn how to install them from tradesmen outside, and some combines it with roof installations, as a big guy you can actually go fully into various aluminium products. Some banks cashier cubicles are made of aluminium so there is no shortage of jobs. Suspended ceilling? I am beginning to feel you must have been seeing a lot of construction sites using it, which shows there is a big market also. I think is faster to learn how to install suspended ceillings and make money quickly.
And if you are thinking of just selling, you will still do well but you have to do a lot of footwork, go to ongoing sites, introduce your products.
A good website is
www.askthebuilder.comAlso read my article
http://www.buildingcontractorsecrets.com/2007/09/artesans-in-building-construction.html#links