Googles your friend: https://electronics.stackexchange.com/questions/29029/choosing-the-right-transistor-for-a-switching-circuit
Yep, thanks for letting me know that Google knows the links to electronics distributors. I was more concerned about a more generalised "holistic" selection of a component, and if anyone has more in-depth knowledge about part selection for my intended use...say if noise/rise time/capacitance/gain/voltage drop/sag etc. would effect my preferred outcome.
Google is great for generalised knowledge, but when it comes to specifics, sometimes a human who has experienced a similar set of circumstances tends to more of help.
If you wanted to know about the sub uS electron transfer in the c-559 chain and its implications for synthetic modelling of biological energy systems, you'd be best talking to someone with a doctorate in the area of study, who will give you ideas/places to locate information...they usually wouldn't respond "Google".
Is the schematic shown what you intend to build? If so, the MMBT3904 shown would probably work. What part number IR diode are you using?
Thanks, I felt they seemed a decent choice (price, availability and they seem to have the correct specs).
I am using TSK5400S side looking LEDs:
Datasheet for Vishay TSK5400S LEDs