Let me answer some of the important queries I got:

Does domain name makes a different in SEO?

Keyword in domain. This is quite hot. Importance Level: 5/5. This will send a very strong signal to search engines that you are highly dedicated for this keyword.

Shall I buy keyword rich domain name for SEO?

Do’s: This helps a lot. You can always a brand website and a website related to the services. In case of a games site if you have keywords like games in the domain, nothing better than that. Search Engine gives huge importance to this. Try and accommodate the most important keyword that represent your industry to it. In case of games industry have games in the domain name. In case debt have debt in it. Also you need to consider the long run of business and brand in your mind. Anytime you can change the domain name, 301 redirect it to a new domain or a folder to get almost all the values.
Don’ts: Don’t make it look spammy with many hypens and keywords. Keep it brandable with the right keyword in it. Example: GamesSuccess.com instead of Free-Online-Games-Success.com

Should I register my domain for 10 years? Domain Length helps SEO

Renewing the domain for 5 to 10 years can helpn building another positive signals with Search Engines, this lets the search engine knows that you are serious about your business. No search engine’s have ever claimed that though http://searchengineland.com/do-links-from-expired-domains-count-with-google-17811 (Read the last line by Matt). Even if this is a positive signal but may not be that strong a positive signal. For me any positive signal is a useful, so I might consider it.

If site age is good, shall I purchase a old domain, then do I get the value of it?

Yes, purchasing a old domain is good. You need to remember few things to get the maximum values. When you buy it, keep everything same including the registrar, then change the design, let it become stable for 2 to 3 months. Then change the registrar and again let the whois remain same for next 2 to 3 months. Then after 2 to 3 months change the whois. The more gap you can make, it is better. The most important thing is the content change. If the content changes drastically then you might discard a lot of link value and old value associated with the domain for that topic. If the purchase is with a good intention and is proved to Google through all the above signals in a very natural way then you can get the maximum value out of your old domain purchase. Search engines are aware about old domains for SEO as a strategies and must have taken all possible steps to stop manipulation. Make it look natural is the key. Even I will suggest to redirect a domain to a folder. Example, if I want to start a finance domain, I can buy debt, mortgage and insurance sites and redirect it to mynewdomain.com/debt/ or debt.mynewdomain.com, mynewdomain.com/mortgage/ or mortgage.mynewdomain.com etc.

If I change the registrar of domain then did I continue to get the same value of old domain?

As I mentioned above, if you change everything at once including the content then it can be a little problem. So be careful, make the change happen in one to 2 years span to make the best use of old site. But if you are just changing the registrar and keeping everything else same, nothing matters, its a common thing among webmasters. I am sure that Google does keep the history with it for future checks.

if I purchase a old domain and redirect it to my new domain then did i get some value of it?

If you do that too often, it is bad. If you buy and redirect to a new domain which has totally different content, you may be discarding some of the links (if not all). The best will be to redirect to a folder and keep the old content live.