The best Idea!
If you are serious about sign making. Contact your local mill. You can also find great prices on Craigslist for lumber that people are milling in your area. The best wood in my opinion is Either Redwood, Eastern Aromatic Cedar (Cedar chest Cedar), White Cedar, Or Cypress. All of these woods are very soft and carve like butter. In addition, They hold up very well outdoors because of the natural properties that they have. Redwood is a bit pricey but the other woods I mentioned, depending on where you live are very inexpensive. I use Aromatic Cedar and I pay .75 a foot for it and the guy even delivers it to my shop! You will also need to invest in a surface planer because your local mill wood is usually "rough cut". A decent planer runs about $299.00 to $399.00 depending on the brand you buy.
Hope this helps.