I'll address the easiest question (#2) first.

Clean clothes are overrated. Personally, I don't bring extra clothes simply for the pleasure of cleanliness, because clothes tend to be heavy and they add up fast. The only exceptions to this rule are clean sox. I usually bring one extra pair of lightweight wool hiking sox and two extra pair of clean liner sox, reserving one pair of clean liners to wear in bed.

Wet clothes are to be avoided at all times, if humanly possible, but keeping your main clothes dry is a far better strategy than bringing a full change of dry clothes.

Lastly, with lows in the upper 40s it should be pretty easy to get by with just sox and underwear in your sleeping bag - as long as your bag is rated for 35 degrees or lower and you have some insulation under you.

Now for question #1. With daytime highs in the upper 60s, the act of hiking with a pack on should keep you pretty warm during the day. When you stop for rests you might want to pull on a light windbreaker jacket. For pants and shirt, I'd recommend all synthetics, like nylon or polyester. These tend not to absorb moisture and dry quickly. Long sleeves can be pulled or rolled up when you get warm, but short sleeves can't be lengthened when you get chilled.

Because getting cold is an issue for you, the best advice I can give is to think in terms of layers. It is always easier to put on another outer layer, compared to adding an inner layer, because with an outer layer you dono't need to take anything off while putting it on. Always bring a warm hat. You'll probably want gloves, too.

Put on another layer somewhat before you actualy feel cold, especially if you can predict you'll need it soon anyway, as when you have finished setting up camp and your activity level drops, and when night nears.

There's no need to drop a ton of money on new clothes, especially not until you discover how much you love getting on the trail (or don't). Take a good look at what the expensive, specialized clothes look like, then go find cheap equivalents. You can even find useable fleece jackets and such in thrift stores. Target or WalMart will have adequate cheap knock-offs for many items. Save the Patagonia for when you are a fanatical hiker.

These answers should get you started. Others can add more details.