I believe Dell has stopped producing LCD TVs. There is currently only one Dell TV available on Amazon--the 26" model. You might try searching other vendors who specialize in used TVs.
To find if your flat screen is compatible with HDTV service you need to know specifically what kind of TV you have. Most recent TV's are. http://products.howstuffworks.com/flat-panel-tvs-and-hdtvs-buying-guide.htm
Sony carries a very popular 60 inch SXRD. Prices vary and a used one can often be found on Amazon. If you want to check the quality, simply visit a store and compare the actual picture in person.
You can buy television wall brackets at any hardware store like Ace or Home Depot. Or you can also purchase them at the store that you purchased your tv from.
Mounts for plasma screen televisions can be found at many different places. Here are some websites where you can find a mount: www.bestbuy.com, www.amazon.com, www.ebay.com.