As a 20-year-old looking to invest in the near future, I love the initiative you have.
A.) I see the trend a lot that investors start with SFH when they are a newer investor because it's easier to get into. Then after they amass a large enough down payment, they 1031 into a bigger MFH and then repeat that for bigger and bigger MFHs. This is kinda the "rich dad" strategy I guess you could say, and makes sense to me. However, if you had the resources and are willing to commit, I'd say you could in theory skip the SFH all together and upgrade but knowledge may be what kills you in that endeavor, so a savvy, veteran partner would be a nice addition. Do you own number crunching though, and find what makes sense for you and your backers.
B.) As for what you can buy, depends on a lot of things. If you want to get appreciation like some investors do, you have to be very picky and identify areas that have growing populations, strong job markets, etc. If all you care about is cashflow, you can try to find a Midwest cash cow somewhere that goes above the 2% rule. They each have their trade offs, where the cash cow may only be worth a dollar more than it is today in 10 years, and the appreciator may only ever break even on cash flow. At the end of the day, you make your money on the purchase, so find a good deal that fits you criteria AND stick with your criteria once it's set.
C.) Make connections with wholesalers and PMs where you're looking to invest, they can pay for themselves many times over if you make the right friends.
Best of luck!