I personally play lots of fast units backed by strong ones, exactly what you were considering getting. If you were planning on making quite a fast list, then you could try getting some old deffkoptas from ebay as they are really fast and are quite effective and flexible, but they are expensive so that is why if you are getting them, you are better off on ebay.
You could have bikes and deffkoptas as an escort unit for your transports and have them take out anything that could harm them. If you wanted more support fire from the back, you could try flashgitz or killa kanz or mek gunz. Also, out of more nobz or meganobz, I would go for the meganobz.