If you're have a limited budget you don't need a bicycle computer. You do need to be able to do at least flat repairs, which means learn how to fix a flat before you ride more then a mile from home. If your not sure on how to fix a flat have your dad, brother, friend, or You Tube to show you and then practice practice practice till you can do it fairly easily.
I recommend a pump and not a CO2, yes CO2 is fast and giddy to use but you have a limited air supply, with a pump you always have air, no carts to buy or throw away so your long haul costs are less. A sturdy long, lasting true high pressure, easy to pump pump is the Topeak Road Morph, a bit large but works far better and easier then small dinky mini pumps. The Topeak Road Morph cost about $24, and the Road Morph G (G stands for PSI gauge that is built into the pump) will cost about $30 on Amazon.
The other thing you need is tire irons, many will work but some could snap especially in cold weather or using on tight fitting tires, so I recommend steel core irons like either the Soma or the Park TL6, both are steel but covered in plastic to protect your rims.
Patches is a personal thing, I've been using glueless patches for over 18 years and never had one fail, others hate them; but the haters simply did not do the proper preparation. If you decide on trying glueless repost here and ask how BEFORE you buy them.
Then carry a spare tube in it's box to protect the tube from punctures while in the saddle bag.
ID with emergency phone numbers, insurance card if you have one.
The above is the minimum you need to ride 20 miles. Getting a mini tool is a great idea if you know how to do your own repairs in the field, otherwise skip. I recommend the Park MT3 tool, but again it's just an opinion and there are plenty of other good tools. Money is great too, but going 20 miles it's not really necessary. Cell phone? I never carried one for 40 plus years, only started carrying one in the last 5 years on the bike, I don't think it's a necessity but if you have one take it with you but put in a plastic zip lock bag just in case it rains while your riding. I once rode 158 miles from Bakersfield CA to Santa Barbara CA without a cell phone and the areas that I rode in were pretty desolate.
