Use of home equity toward prchase of 2nd home

Curently own home, mortgage free.  Have been advised I can use the equity of current home toward purchase of dream house (listed at amazing price), with the intention of selling current property (with commercial value) at later date when more valuable.  Is this a practical undertaking?  What percentage of current property value would be offered as equity?  Would I essentialy be paying interest on a HELOC on currently owned property while paying off a mortgage on new home?

Thanks for your help,

Ed