You should have a good idea of every expense you have in your primary home, then add the STR specific:  (These are rough estimate numbers, depending on various factors)STR insurance - a special insurance that covers both the property and your liability, could be $2~4k in areas without natural disasters, could be more than $10k in some other areas.cleaning fee - approx. $50 per bedroom, so a 4-bedroom house will cost you about $150~200consumables - bathroom supplies, kitchen supplies, beverage supplies: $20~30 per week, depending on how much you providewear and tear - towels, linen, kitchen utensils, some furniture: $1000~2000 per yearRepairs/replacement for things that guests break or steal:  $1000~2000 per yearAlso, if your personal use is less than 14 days a year, the entire property's expense is deductible against STR income, but if you use more than 14 days, then you will have to calculate the proportion of how many days it's rented out vs how many days you or your family uses. 

I would budget maybe 20-30k depending on how far you are going down.

I personally think it really depends on how much value will it add to the property and if the town will allow it. 

It really depends on where you want to take your real estate investing.

I think it would, depending on the celebrity of course. 

It will be highly dependent on the LTVs, but 2nd's on investment properties get sketchy. 

Depending on the type of commercial you want to get into my suggestion is to go work for a company that does commercial real estate as learning from people who do it and getting paid at the same time is not a bad strategy 

@Chris Seveney's suggestion could be a really good solution depending on the numbers. 

It really depends on your bigger picture and also what you are hoping to accomplish in the future.

Since I'm not a lawyer, there might be a specific reason, depending on your situation, why that might be useful.On the other hand, I've seen crazy large structures for small portfolios, and there are many attorneys willing to charge you for the help of setting them up.