1. What is the minimum amount that I can invest?
我最少应该投资多少？
还能这样说：
How much can I invest at least?
What is the minimum investment?
应用解析：
invest in 投资于；投入；
invest with 授予；使带有；使笼罩着；被……所包围
2. We plan to buy a new property as an investment.
我们计划投资买套房子。
还能这样说：
We are thinking of investing in a new property.
A new property is our investment.
应用解析：
go according to plan 顺利进行，按计划行事；
plan sth. out 策划或详细安排……
3. They decide to participate in the financing of real estate.
他们决定参与房地产投资。
还能这样说：
They make a decision to invest in real estate.
They want to take part in the investment of real estate.
应用解析：
participate in a discussion 参加讨论；
participate in profits 分享利润；
participate sb.'s sufferings 分担某人的痛苦；与某人共患难
4. It's the best time to invest now.
现在是投资的最佳时机。
还能这样说：
It is wise to invest now.
It's to your advantage to invest now.
谚语：
Wealth is best known by want.
人穷方知钱可贵。