How big a plot of land? That might affect the sorts of buildings you go for.
Would I be correct in assuming that you're using the actual downtime rules from Ultimate Campaign? If so, please let us know how much capital you already have (if any).
The Ultimate Campaign downtime rules area probably the most applicable rules I can think of for what you want. They tend to be unwieldy though, and profits aren't quick - IIRC you'll get an effective return of about 10% per year. In real life this would be good, in-game it's slow but consistent. There's a lot of spreadsheet management too.
The advantage of the Ultimate Campaign rules is that once you've got a business earning capital it becomes a lot easier and cheaper to expand that business, or start another business, or recruit henchmen etc. Something like a Dock (which can produce Labour, Influence or Goods) provides a solid foundation for a business empire.
Starting by buying a Waterfront or Stockyard (both able to make decent amounts of Goods and Labour) should make your 20k investment go further - similar to investing in crafting feats.
Were you planning on a single big building, or more or a village/complex? Because 20k gold will go a very long way once you've got an initial building churning out capital at half price.
Let's plan a simple complex, suitable for a Cayden-worshipping Ranger. All listed prices are UC example buildings, and can be halved if you earn the capital instead of just buying them:
Waterfront 2680gp
Barracks 3770gp or Guildhall 2660gp
Inn 2130gp
Brewery 1450gp (because Cayden, c'mon)
Farm 2090gp (to supply the brewery!)
Shop 550gp
Shrine 270gp (just stick it in the Inn!)
2x House 1290gp each
All the above (going for the more expensive Barracks instead of Guildhall) costs a total of 15k. At full list price, not including any discounts. Leaving 5k left over to hire employees, start a mercenary company, throw a massive drunken party, or whatever you choose.
And, just to make things even easier for you:
Waterfront: +80 gold = 9gp per day
Guildhall (I went for this rather than barracks): +39 gold = 4gp per day
Inn: +52 gold = 6gp per day
Brewery: +36 gold = 4gp per day
Farm: +33 gold, +4 influence = 4gp per day, 1 influence per day
Shop: +7 gold = 1gp per day
Shrine: +1 gold, +3 influence = 1gp per day, 1 influence per day
House (x2): +18 gold, +8 influence = 2gp per day, 1 influence per day
That lot nets you 22gp per day, not counting the benefits of staff. (It's actually going to be closer to 30 because I rounded down to whole gold.)
Adding in staff (dockhands, guild staff, inn staff, brewers, farmers) adds another 4-8 gp per day.
And you'll still be able to have a massive drunken party.
22gp per day? That's pretty damned good, Chemlak! Thanks for adding it all up. That's 8k per year, or just under 2 years until you've fully repaid the initial investment and everything on top is profit. Because there's basically no upkeep if your character supervises the operation personally.
If you've got a campaign where downtime is likely to be measured in months to years, the UC rules actually seem to be more-or-less cost neutral. Having a small settlement (and dozens of employees) on your payroll gives you a lot of narrative power too.
If you're able to able to generate capital (and thus halve the price of rooms and teams) then a 4-5k investment will get you something like 50 Cavalry and 50 Elite Archers, along with the bureaucrats, laborers and craftsmen needed to run this small army. With a permissive GM and a bit of time and money you can be a warlord.
Donate all the money to charity. Funnel it all out, though, don't make it public.
You're not going for fame, or love of the masses. You're doing it because it's hard on a peasant NPC out there, and you're character is a good person.
Not materialistic, focusing on power and wealth.