1. Taxes: about what would taxes be be on a 30 grand boat in NY?
Will you be keeping the boat in New York? If not, generally, you will owe taxes where you keep the boat, not where you buy the boat. With regards to taxes in New York - it's the same as sales tax, and it's based on where you live. In Rensselaer County in NY, where I lived when we bought our boat, sales tax was 8 1/4% on the purchase price. One tip - if you buy it elsewhere and bring it to New York, if some time period (I think it's 6 months) elapses between when you buy the boat and when you enter New York waters with it, you only pay sales tax on its current valuation. If you can show other boats listed for sale for cheaper, you can reduce your tax burden legally. The state you buy it in, however, has to allow you to NOT pay sales tax upon purchase. We bought ours in Maryland, and it was on the hard, so Maryland let you sign a form stating that it would be leaving the state for transport to another state and waived Maryland sales tax.
With regards to title - in New York there is no title on boats. There is registration, and it's done at the regular DMV. You bring your bill of sale and then fill out the forms and pay the sales tax. If the boat was already documented by the Coast Guard, you can pretty much just go to their website and fill out the forms to mail in, along with a check for something like $100, to transfer the documentation to your name. If it hasn't been documented, and you want it documented, there are a bunch of steps to go through and you may be best served to get a broker to walk you through those.
With regards to payment - I've never seen a broker take credit cards, so you will probably be paying cash/check no matter what. Your final price will be your final price.
There are no special aspects of changing the name of the boat except for the name change ceremonies. Do a search on these forums for more info on that.
With regards to sea trial, inspections, etc. do a search on these forums - there is a ton of information on these areas here already.