Hi skuzy,
That's an amazingly broad question and can't be properly answered in anything other than vary general terms knowing something about you.
An offset account can be very useful, if used in the right manner. The right structure for an offset account will depend on whether or not you own your own home as well as the IP, how much equity you've got, what your plans are, etc, etc...
Fixing interest rates might be something you'd consider. Anyone in the last 5 years who's fixed their rates will probably be smiling about it now. Fixing now may or may not saving you money in the future, but you will know how much that loan would cost. The decision would depend on your cashflow.
Variable rates are useful for flexibilty to make extra repayments and make redraws when required.
LMI or not LMI? Using LMI can accerate your portfolio building, but there are pitfalls.
Using a professional package can be a cost saver, but a basic product might be just as effective depending on the loan(s) required.
Different lenders treat various types of trusts differently. A trust can be a useful investment tool but it can be costly as well. It depends on what your needs actually are.
Most importantly if you're planning on building a substantial portfolio is that things are structured in a way which enables you to grow quickly. You want your portfolio to be flexible enough to get access to equity when you need it, but to be rigid enough to endure rising interest rates. There's no reasonable way to advise on this without knowing a lot about your circumstances. Plans also need to be reviewed and adjusted on a regular basis.
The only way to get a reasonable comparison of what's available which is based on your needs it to discuss your situation with a good broker. Give myself or one of the other brokers on this forum a call.