S.!
As an interior designer, personal home flipper,and real-estate worker I can tell you that you are on the right track! Things that "cost-effeciently" up the value of your home are many of the things you mentioned above!
The things that most often sell a home are as follows:
-Updated kitchen (which can be quite costly, so if yours is fairly good, skip this one unless you can do something minor with big impact)!
-Updated bathrooms (sometimes quite small repairs/additions are worth their weight in gold)
-Fresh-appeal (fresh paint, flooring, light fixtures, etc)-VERY IMPORTANT!!!!!!!!!!! Makes the house seem clean, open, airy, and homey!!!!
-An inviting atmosphere (front door, curb appeal/yard, entry way on outside and inside the house)! VERY IMPORTANT!! This is the total FIRST IMPRESSION of the house!!!!!
-Clutter-free, quaint, clean, simple home (when showing, find/use nice furniture even if you have to borrow it for a bit, not college "left-overs, as this is a big turn-off! Better to have virtually no furniture than old/"bachlor"/ratty furniture!)
Where to start: With what you listed as possibilites, that would be hard to choose based on not knowing your house's character! BUT, you can find carpet and paint cheap and replacing it is always a MUCH better option if it at all looks dirty, dingy, or old! Home Depot & Lowes have great options and you can get them quite cheap when you catch them on sale (esp. holiday sales ie: Memorial Day, Thanksgiving, Labor Day, etc.)! But, just a good carpet cleaning can also do wonders if you just can't justify new carpet and you can rent cleaners from many stores near you for cheap and by the day (do it yourself, don't hire it out and you will save a ton-places like grocery stores, lumber yards, etc often rent the machine and sell the cleaners to go with it)! Be warned though that it is a water cleaner so it will take time to dry (don't do it right before potential buyers arrive)!
As for the roof, I would go with 20 as it will be much cheaper and not much of a selling factor!! Plus, with Texas weather, after the first hail storm, the new buyer will collect and possibly replace anywhere. A new roof is a new roof no matter what-it is always marketed as "new", not " a new 30 year roof", so go with the lesser of the two!
And the door: look for sales and building outlets/surplus stores! You can find great buys there and quality is usually just as good or better than normal re-salers!! Entry door is very important but if the style is fine, consider painting????
And the bathroom: how out-dated is it? Can just a light fixure or some "little" thing do the trick or is it soo "old" that it needs more than that? Typically, bathroom fixtures are the same in quality and just some-what change in style over the years. De-clutter it and REALLY Look at it and make the assesment!!!! Can you dress up what you already have? Are there mold/mildew stains you can get rid of by cleaner or re-grouting, or cover? How can you freshen it up?
As far as when it comes to showing/selling it: TOTALLY DE-CLUTTER YOUR HOUSE!!!!!! This makes the house look bigger for the viewing!!! Allow a lot of sun-light in! Make sure furniture arrangment complements the room! Light candels or bake cookies/bread at the showing-smells are an awesome insentives towards a sale that helps "sell" the house to the viewer sub-consciously(and you can serve the fresh cookies to the guests)!
Best of luck and have fun with it! Do a little at a time and it is much more fun, less stressful, and easier on your pocket book!