@Bharath Raj - First and foremost, congratulations on your purchase. Owning the building that you will place your practice in can be financially promising and certainly exciting!
To answer your questions:
Do I need to notify my acquired tenant regarding the ownership change?
It would be good to notify your tenant regarding your purchase of the property. I'm sure they are already aware that the property is up for sale and likely sold. After all, they were probably present when the appraiser, insurance company, and possibly lender came to inspect. If nothing else, making a formal introduction that you are the new owner will be great in getting your relationship with your tenant off on the right footing.
Do I need to do a new lease?
No, you do not. The lease they have in place still holds and is likely transferred in kind to you. This is of course presuming the tenant has a long term lease and is not going month-to-month. If it is the latter, then of course you and the tenant can work out a new lease if both parties so wish.
Do I use a attorney?
Generally, you are not required to use an attorney. But if you would sleep better at night, then by all means, do so. After all, it is always a good idea to seek professional advise if you are uncertain about any factors of investment property ownership, tenant laws, tax matters, etc.