I can invest three to four lakhs every month for 15-20 years. Can I invest only in mid-cap and small-cap funds?
Yes. Just make sure that you are comfortable in doing so. Look at the past of these mid-cap and small-cap funds. Look at what happens to these funds in the worst of times and assess whether you will be fine with a similar situation. But if you are investing for 15 to 20 years, they should turn out to be substantially more rewarding. Also, this is a category of funds where you will have to be a little more active. You can't choose a fund and be at it for the next 20 years. You have to review your selection every two to three years because things might change. The fund manager may leave or the fund may become too big which is a serious detriment for it to perform. So, make your choices, start your SIP and review your investment every two to three years.