If, you are drilling in a hard rock formation like granite, gneisses, decan trap, besalt, schist etc.etc. and size and depth of hole is known, opt for down the hole hammer rig and the following features of the rig may be evaluated carefully;

Design and capacity of the mast.

Torque and speed requirement of top head drive.

Pull down and pull back forces. Feed system.

Type and capacity of Compressor pressure, volume etc.

Capacity of prime mover, H.P., RPM etc.

Capacity of mud pump pressure, volume etc.

Capacity of water injection pump,  pressure, volume etc.

Drill pipe size and storage capacity.

Casing handling capacity.

Jib boom and winch capacity.

Safety shut down system. Cooling  system of the rig.

Lubrication system.

Electrical system.

Whether truck mounting or trailor  mounting.

Accessories as per requirement of  the  rig.

Casing driver / odex, if required.

If you are going to drill a hole in semi-consolidated hard and bouldery formations and size and depth of hole is known, opt for percussion rig and the following features of the rig may be evaluated carefully:

Design and capacity of the mast.

Maximum tool weight.

Construction of spudding beam with stroke length.

Bull reel capacity and size.

Casing reel capacity and size.

Sand reel capacity and size.

Design of jack shaft.

Whether tailor mounting or  truck mounting.

Capacity of prime-mover H.P., RPM.etc.

Type of transmission.

Type and size of clutches and brakes.

Type of sheaves and bearings.

Type and size of leveling jacks.

Accessories as per capacity of  the rig.