I would say MGMAT,GMATPrep and other free tests such as Princeton's,Verita's are more than enough to get a grip on GMAT.
If you want more practice u can buy Kaplan.
But Kaplan tests are not at all representative of actual GMAT.
So the scores u get may be totally different from the score you get in actual GMAT.
On the other hand MGMAT tests are quiet good.But MGMAT SCs are little different from the SCs you get on actual GMAT.Hence in the last few days of your preparation after you are done with both the GMATPreps you can go through the MGMAT forum for GMATPrep SCs.
All the best!

Dont worry about Kaplan tests. Their so called adaptive tests give you chunk of DS,PS,RC,SC,CR at a time. you can figure out how adaptive they are then.(i ahve the updated CATs)
Their customer service too deosnt bother to provide you with proper replies.
Just give them if you need xtra practise on timing etc.

MGMAT is the best!!

Thanks Rohu and Prachi for your advise. I am taking up Prachi's suggestion to attempt the free tests of other vendors. I just registered myself to Veritas and Knewton.