1) The 9% is per annum. You would calculate it to the date of redemption and may not be able to collect it at all until certain notices have been provided. But any amount you would receive, considering your proposed investment, would be negligible. MOST tax liens are redeemed by the owner, mortgage company, or other third party.
2) The tax lien is in a superior position to everything else. There are additional steps you must take during the enforcement process whereby you notify the condo association and just like a mortgage company with interest in a property it can also redeem your lien.
3) Technically you need three piles of money. You need your collateral, purchase funds and then the legal and administrative costs you will incur converting a "Certificate of Purchase" into a "Tax Sale Deed." You can borrow the money you will need for the legal costs as your collateral since you will have it back shortly after the auction. You may also post a surety bond which premium will depend on your personal credit and financial resources, but would run somewhere around $100 per $10,000. In all you will spend $10,000 or more in legal and administrative fees in the rare instances you make it all the way from winning at auction to a Tax Sale Deed.
4) It is very rare to make it all the way to a tax deed. Most properties are redeemed. Most auctions are won by institutional investors seeking the 9% interest. Those investors are good sources for tax liens that are much further long in the process, but you must be working with a great deal more capital. I do not see this being worth your time as any returns will be negligible over what you're earning in a savings account. Your chances of winning the few auctions you'll be able to participate in are low and even if you do it will probably be redeemed.
5) Tax attorneys are not the correct attorney for this sort of venture. There are law firms that specialize in properly making all of the required notices, filing the foreclosure in court, serving this properly, following all the technical rules required to have a valid tax deed issued. They are paper mills and frequently operate multi-state practices from one location.
Bottom Line is the rumors of going down to the tax auction and obtaining someone's valuable real estate for pennies are mostly false. I've purchased tax lien properties but never started at the auction. If you look at the auction results from last year and the year before you will see the major winners of multiple sales. Look them up in court records and see if they're getting close to foreclosing any property you're interested in. Most of them don't want real estate and are happy for an offer to acquire their lien at the end of the process. This can be a savings on real estate, savings worth your time, but it's not a fire sale.