You are in the same position I believe a lot of others are in and there isn’t an easy answer. The first problem is running scale and hi-rail together, some say they don’t mix well together and others will say you can do it but it needs a bit of work on the turnouts to achieve it. I guess the simple answer is choose one of the options that you prefer and see what is available with regards to track. For the scale side you are going to be limited to some extent on rolling stock/motive power unless you become inventive to overcome the associated problems.
I’m sure others will chip in and provide more detail than I can which will be more constructive.
