Long term planning for down payment

[Mar L.]

We are interested in moving and buying a new home within the next 5-10 years. However, we would like to keep our current house as a rental property. We don't have a down payment yet so moving forward, where should we be saving this money? We would likely need about $150K for the down payment. What about plowing money into our current house's principal, and then take out a home equityloan when we need it?  We can probably save about $1-3k per month, with more money in later years since we'll be free from childcare costs.  

Our concern is that 5 to 10 years is a really big range and we really don't know if or when we will be ready. We also have another small rental property that we would like to hold onto since the returns have been good.