Bro your budget is not enough for a complete good sounding system, although it mayb sufficient if u buy chinese stuff but that's not recommended cz they sound like crap. So ur plan should be to install quality stuff in steps.
First step would be to keep ur existing pre and install front component speakers ( recommended Focal Access SG1 ), may cost u around 15k and get a good 4 channel amp for your speakers. Jz is selling a kenwood 4ch amp or u can check market for pioneer 4ch, should fit in ur budget for now.
In second step you can get rear 6x9 speakers and run all 4 on ur 4ch amp (mayb cost u around 10-20k)
Third step would be either a new pre with atleast 3RCA out (10-18k)
Last step would be to get a monoblock amp and sub depending on ur budget.
All this depends upon ur budget cz the combined total for an entry level good system with set u back by around 80 - 85k. If u have sufficient cash u can combine steps.
Hope this helps.