Hi, welcome to the habit, er hobby..). Well let me address your second question first. There is a saying in ponding, "Build your last pond first". What this means is almost everyone usually builds to small at first and ends up expanding or building another pond. I still have my first pond which is 2000 gallons. I wasn't going to make the same mistake again, my new pond is 50,000 gallons.
Now to the second question. My first pond is a garden pond, no bottom drain, gravel bottom for aquatic plants. I usually drain this and clean this every 2 years. My large pond is a koi pond, no gravel and 2 bottom drains. Bottom drains help get rid of any settlement on the bottom and fish waste. Most koi enthusiast will say this is essential. I've been seeing a new technology in koi books of a system they put under gravel that acts like an under gravel filter. This is very new and if your lucky enough to find a contractor that knows about, I would look into that. It is a system of PVC pipes with small holes that is buried under the gravel.
Now let me give advice from experience. My land slopes downward at a 10 to 20 degree angle downward. My larger pond was just build last year and placed in the middle of my land. We get a far amount of rain here in Pennsylvania. I've had to add a series of drainage ditches to stop runoff from going into the pond. It has been turning my pond water brown and messing with my values. Only last week was I able to add fish. Also, upon building we hit an underground spring. My contractor put in drainage pipes underneath the liner that flows to a sump pump. This is needed because water underneath the liner will flip the liner. If you have that much water on your land, I would look into building your pond half in and half out of the ground. Either that or make sure the edges of the pond are raised so no water can go into your pond. There are many ways you can build a pond.

Hope this helps...Craig