Pick a clan(s) that is good but avoids bans a lot, but you also have to like the clan(s), choose a star format, many people go
22333345 or 22233445 or 33333334 (if mono but rare that someone does this) or 22233355.
Now pick the best cards you can afford that are not banned in that clan(s)
Well depends on how much money you have access to, but a general good idea is to check the election page. The election page lists all the characters that are used the most in elo and generally lists 4 for each clan. Look at this page if you want to see what the most popular characters for each clan is and pick 2 of them that dont have many characters hovering at the 5% mark. Generally speaking its a good idea to only have one or two 2 star characters then focus on having a balanced clan. You should aim for having at least one half of your deck to have a soa/sob or attack manipulation bonus, clans that have these bonuses tend to have lower damage but can reach high attack with little or no pills, so pairing them up with another half like junta or bangers for damage is not a bad idea. Really its all up to your play style but achieving a good balance is ideal, doesn't matter how good your deck is if ever other hand you draw is weak.
20k is plenty if you already have a half deck, here are a few half decks that would be able to compete in ELO for under that
Uppers - Dorian, rubie, oxen, samantha. - 11.5k clintz
Sakrohm - Sigma, Petra, Murray, na boh - 9k clintz
Roots - Noodile, Arno, Ojibway, Rico - 12.5k
Montana - Mona, Edd, Spiaghi, Ottavia - 17k
Freaks - Bogdan, Olga, Esmeralda, Piotr - 14k
All stars - Marina, Stacey, Jessie, Randy - 14k
Those are a few examples of some of the cheapest half decks in UR that still contain the best that the clan has to offer. You could stick with mono piranas but i recommend using 2 clans since one clan has a few weaknesses that can be supplemented by another clans strengths. Piranas tend to lack damage so finding another half deck to help this is ideal.