I get this question asked of me a lot, and the first thing I ask is "Are you doing this to save money?" Because if that's your reason for trying to make one, you're in for a shock - it's now cheaper to buy a new one that put one together from separately bought pieces.
Seriously, do your homework. Here's your shopping list: Primary mirror, secondary mirror, focuser, finderscope, at least two eyepieces, wood, tube (range of choices so depends) screws and bolts. The purchase of the mirror cells, both primary and secondary, and the spider, is optional, as it's not all that hard to make them, if you're handy.
Take this shopping list and browse the various online stores and write their prices down. Compare them to the ready made units. Hmmm....
It always amazes me how many screws and bolts you need. Whenever I take a scope apart I seem to end up with half the scope being these things!
I love building scopes, but I've been through too many people who rush into this without really considering the current state of play in the market. Too mnay people have said to me "Why didn't you tell me it was going to be like this! If I knew it was going to be this much work, and cheaper, I would have just bought one!" so I don't hesitate to lay it out bold and bare.
However if others are reading this also considering making a scope, and perhaps money is not the issue for them and they would like the experience, then go ahead! It's also a great way to customise a scope to the way you like it.
Another option not often discussed is buying a second hand one and refurbishing it. Sometimes there are some beauties out there languishing in back yards that people don't know what to do with anymore and you can get a bargin. But you need to know what to look for to see past the muck, unfortunately, and that only comes with experience.
To answer your questions directly:
1. Size - and 8" or 10".
2. Cost - already talked about that, but the dob I own now I made in 1997 and in the end it cost me $1050 once I'd paid custom fees importing stuff (hey, it was harder to do back then!) which in those days was just $50 more than an off the shelf 8" (yep, things sure have changed in ten years...)
3. Plans...ummm I'm bad and don't work that way and when I teach other people I tend to focus more on the principles of what you have to do to make it work rather than what exactly to do. When you build your scope you can solve the issues at hand in a number of different ways (more than one way to skin a cat, as they often say in the atm circles) and it just depends on what you like and what skills you have to acheive them.
4. Things I did that I wouldn't repeat - dont buy a cheap focuser!!! Buy a good one, pay the bit extra, you wont regret it.
5. Where to start - the primary mirror, and it's focal length. Everything else will flow on from those two parameters.
6. Advice - I think I've already given it.
.
.
..grind a lens for a refractor, make your own mirror and even silver it
.. just goggle ATM or the like you will be busy 
