On my Buy to let mortgage, I had to place a deposit of 30%. The interest rates were slightly higher than a traditional mortgage also.
I think what you're planning to do is VERY wise, you will be laughing in the future once you've got 2 properties paid off (the rented one should pay for itself anyway, so there shouldn't be a financial burden on that property, other than the initial deposit.).
I would strongly recommend you go for an 'interest-only' mortgage, this will ensure you are not paying more than you can afford on a monthly basis (as you have 2 mortgages) and if anything would happen with your current income, you will not be hit as hard and still be able to cope with the payments. An 'interest-only' mortgage should have very low monthly instalments, your rental income should cover it and give you a small amount of profit on top, allowing you to make over-payments at the end of the year (a lot of mortgage companies allow up to 10% overpayment without being penalised).