Not sure there was a stock 20" truck wheel in 2005, so are they OE wheels off a newer truck or aftermarket? If aftermarket with 305s they may be too wide for skinnyer tires. Just something to look at. Nothin wrong with 20s inherently though. Best size for an economical stock type ride would be 285/60 or 275/65 like you mentioned. 275s not as popular so typically cheaper to purchase.
Get E load rating, most of the 20s in the above sizes will be 3500-3700lbs rating which is sufficient.
Brand/style? That's like asking what the best thing to eat for lunch is!!
Tips to find cheap tires....check Craigslist for OE takeoff tires and/or wheels. Good way to get new tires for around $100 a piece. Amazon is good too. Watch for deals. Last set was new 275/20 Firestone MTs for $760 free shipping.