I generally recommend buying a notebook that already does everything you want. Even things like adding RAM can be a challenge several years down the road as RAM technologies advance and current RAM goes out of production and becomes hard to find and more expensive to. So if you want 16GB, buy it now. That said, 8GB is a big chunk (only 4GB would be a mistake, IMO). For programming, you really don't need more than 8GB. Of course need and want are two different things. With only 4GB, a huge percentage of that will be taken up by Windows itself and your security programs. That does not leave a lot for your other running programs, forcing the system to spool out the slow hard drive much more often. With 8GB, that is plenty of room for Windows, your security programs and your apps.
You can add a Bluetooth USB adapter, but that will use up a USB port. BT is used for things like external keyboards and mice and maybe an external speaker. HDMI is used to connect an external monitor. Every notebook lets you do that and most are with HDMI or Display port. That should not be a problem.
USB is frontwards and backwards compatible so you will be able to use USB 2.0 in the future - you just will not be able to take advantage of USB 3.0's faster speeds. But I say buy current. USB 3.0 is here now.
I cannot suggest the right size for you. If you listen to me, I would tell you to get a PC, not a notebook, and at least one, maybe two 24" monitors, a full sized keyboard and mouse. Do you really need a portable computer? People who travel a lot typically don't like lugging around big, heavy notebooks so for them, 15" screens may be too big. 17" screens are much nicer, IMO, but of course those notebooks are much bigger and weigh more too just because of their size, but also because they typically need bigger batteries too.
And speaking of the HDD, I would also suggest a SSD instead of a HD. SSDs are MUCH faster, lighter, consume less power, generate less heat, and with no moving parts, should provide years more of trouble-free service.
So again, while it will cost your more initially, it generally is better and cheaper in the long run to buy everything you need in the beginning.