This is my business basically (and have been doing it for about 20 years).
Suggestions:
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1) Choose the domain first and then brand with it. Not a good idea to brand first and then to buy the domain name
2) Go with .com unless it's clear that if you pick something else you stand a chance of getting the .com later (That is difficult for someone who doesn't know the business to figure out. [1])
3) Biggest mistake you can make is getting to married to a particular name and not have any alternatives. You will almost certainly telegraph that and pay more.
4) Second biggest mistake is getting anything but .com and assuming it doesn't matter and you can buy it later. In most cases (there are exceptions) it does matter. And the price of the .com will only go up after you are established and locked into the .whatever
Pecking order of domain sellers in terms of difficulty of completing a purchase.
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A) Large corporation, say Google or General Electric. Typically won't sell at any price
B) "Old School Domainer" - Typically a gambler who will bet you will pay a zillion for the name if they hold out.
C) "Domainer" - Will sell but wants a decent price but tends to be somewhat rational.
D) Small business owner - Will negotiate but you won't get raked over the coals.
E) Professional domain site such as buydomains.com. Pricing is typically very reasonable even the list prices on the site. My go to place when I have to buy domains for others. Prices are stable they don't reflect who the buyer is ie they don't size you up.
F) "Little Old Lady" - Happy to get what they can.
[1] As a general rule I'd feel comfortable if the .com was owned by a small country store that was lightly using it than if it was owned by a domainer and/or a large corporation or the type of buyer that typically won't sell at either a reasonable price (I didn't say "cheap" by the way) or at all. [2]
[2] Corporations (large and even small that aren't owner operated) have typically little interest in selling because nobody personally benefits from the sale. At least a "domainer" gets to pocket all of the money typically.
Edit: Added another seller to the pecking order.