Mortgage pre-approval is a lender's written indication that — based on your financial situation — they're willing to lend you a specified amount, subject to conditions. It's not a guarantee, but gives you confidence to bid at auction or make an unconditional offer.
Yes — each pre-approval application creates a "hard enquiry" on your credit file, which can reduce your credit score by a small amount and is visible to future lenders for 5 years.
Yes, and this is recommended. Having pre-approval before you start inspecting means you can move quickly when you find the right property.
No. Pre-approval is conditional. The most common stumbling block is the property not valuing at the purchase price, particularly if you paid above comparable sales.