1. Is it a Full and Final Settlement Offer sent via e-mail ? Make Sure it is so before you pay and get all the details like "Settled" A/C Closed etc to be reported to CIBIL in this Full and Final settlement letter for A/C XXXX XXXX XXXX XXXX from HSBC.
2. OK if he is genuinely from HSBC, then send him the DD favoring HSBC Credit Card A/C XXXX XXXX XXXX XXXX [This confirms you are paying to your A/C only]
3. "Settled" implies not fully paid but something was mutually agreed upon.
4. Affect on Home Loan - Depends on your current status. If you are salaried and have Rs 1 Lakh per month salary and availing home loan EMI of Rs 30,000 per month, bank may charge you extra say 100 bps [1% ] than normal home loan rate of interest. They may finance only 80% or lesser value of the home and ask you to cough up for 20% of Home Value as upfront. All depends on your current status [Financial Standing] and how well you can negotiate.
5. Once the HSBC thing is settled, get your Father, Brother, Wife etc to lend you a helping hand to get out from the Credit Trouble, if they have clean credit history and are willing to sign as Guarantors, Banks will do it in your favor. However, don't jump. Stabilize your Financial Standing and then take a well thought decision so that everybody around you is HAPPY.
Best