0603 isn't too bad to solder by hand (I won't do 0402 or smaller though).
SOT23 is probably a good guideline (for diodes too, not just transistors); there are some SOT323s that are smaller that are a pain.
I would avoid certain SOT23-6 parts because it can be very difficult to determine which way the package is supposed to go. (For some dual MOSFET packages it doesn't matter.) We had one where there was a slight bevel along one edge. Grrr.
I would also avoid SOD123 because of the backwards nature, if possible. SMA/SMB/SMC aren't as much of a problem.
And avoid those cylindrical diodes (LL-34 / MELF) like the plague! they will roll off the board.
Let me start with your last question, I would highly recommend NOT soldering components on both sides unless you have a really good reason to. I have made a few boards with double sided soldering and it became more of a pain then anything.
As for the routing itself, my answer here might help you out some, but I will elaborate some for your specific circumstances.
Part density can become an issue when you are hand soldering, but it is difficult to give exact numbers as everyone will be different with how comfortable they will be soldering and what parts it is. As a minimum you will probably need 2 soldering iron tips width apart. This will give you the room to get the iron in to work on one part while not hitting the other. You might also want to take into account the angle that you like holding your soldering iron at as you won't want to be resting your iron on another IC. If you have a shakier hand then you might want to see how much your hand shakes and space your parts at least as far as the tip of the iron moves as you are shaking.
I would also avoid running traces between legs of SMD components. A lot of people will do it just because it passes DRC, but if you are hand soldering with no soldermask, it becomes very easy to accidentally bridge to the trace.
It is also helpful, but not required, to bring your traces out straight from any ICs and then after a little room branch them to the direction they need to go. This will help you line up your IC properly as well as to be able to get the solder in place easily.
And finally, going from through hole to SMD, you will find that many of the tricks that you can use with through hole just wont work with SMD. Things like having no vias because you are using a through whole component to jump to the back side, instead you might have to go back to your schematic and change things around to limit the number of vias used. Also you can usually run traces under through hole items, but this may complicate things more with surface mount.
Overall, just practice and you will pick up tricks just like I am sure you have picked up tricks with through hole.
Best Answer
When soldering is done by hand there are lots of aspects that factor into how fast it can be done. Here are the largest factors for me when I am soldering:
In general though, I would say it takes me about 45 seconds per part to place the parts on the board. The board can then be baking while more boards are assembled.
Where you have to be careful in time estimating is the time it will take to check for shorts and fix them. There will be errors, maybe just a couple out of the whole batch, but there will need to be more testing time when soldered by hand.