Many ways to get initial money - corruption, governorship appointments, raid enemies, etc. Then build up your wealth by investing - get businesses in every city, upgrade your villages, get some latifundia and upgrade them, etc. Eventually you will have a high income and saving up a million will take just a few weeks.

If you go emperor before you are rich, it will be harder. Depending on how your takeover plays out, the economy can be in disarray, there are a ton of lords that you will need to keep happy, and nobody will be paying for your armies anymore. So I'd suggest saving up a lot more than the minimum before trying to take over the empire.

you can give out loans max 500k of loans, also if you conquer a fort or city or convince someone to receive some fiefs it can turn really profitable, you will also receive the salary of your rank, legate being the highest, but dont expect to get alot of money since its like 7k