Hi Raghu,
Since you have good experinece in MM/ SD Domains you can understadn related SAP modules easily, not a problem.
Coming module selection SD / MM / CRM all three are very good modules no doubt at all. you are best person to take a decission on that.
But pls consider the below points before making decisson & go a head. dont depend on any ones decission, take it as an advise only & finally you take a right decission what ever it is its going to be the best.
1.Pls Read online docs & help.sap.com / scn related to sd /mm/CRM module try to understand the overview & scope of those modules which will helps a lot to know about it in detail.
2. Usually people who came from sales & procurement domain will choose MM & SD modules there are lot many people on both modules.....
3. Some web sites providing pre recorded Audio & video training manuals ( you can get free of cost by searching it ) for all modules try to find & know how those modules are related & how you are connected with those modules based on your knowledge * exp. what is your understadning levels on that ...
4.Before going to vertified training by spending huge money learn your self with the help of on line docs & videos & advises from friends / professionals.
5.Follow SCN and try to see how the solutions are provided.
6.Certification is one of way to enter into SAP, but it does not guarantee the job, for your information most of the people dont have certification & still they are leading ..but certification will helps you.
7.Since you told cant afford certification dont worry there are many institutes available with out certification try to enquire on that.
Hope i have answered all your queries, Based on your R&D proceed furthur.
Don't lost the hope & don't get panic, initially these steps are common for everyone be bold,& confident and plan in a cool manner based on your available resources to make use of time effectively.
wish you all the best.
Regards,
N.S.Rao