I presume that you are asking for service tax liability under reverse charge mechanism.
Hence, your answers are
1. If you, being a body corporate, receive a works contract (material + labour), then first you shall see whether the service provider is an Individual, proprietory concern, HUF, partnership firm, LLP. If yes, whether such service provider is registered or not, the 50% service tax liability on such contracts shall be borne by you.
For valuation (under Rule 2A of Valuation Rules, 2006) for such works contract under composition scheme is -
a) Original works (New construction, machinery or plant or structure making) - 40%;
b) Repairs, maintenance, etc. on plant, machienry, structure - 70%;
c) other - 60%
Hence, I believe you must be falling under (b) as repairs to machinery. Therefore, you will have to pay @ 4.326% (12.36% x 70% x 50%) on the total invoice value.
I again repeat, it does not matter for you being a body corporate whether the service provider is registered or not, you will have to discharge Service Tax liability on your own by your own calculations.
2) there is a specific defition of works contract has been given, which does not include the term painting, but it includes renovation. It is upto you to decide whether it falls under the works contract definition or not.
According to me, if it pertains to renovation then it shall be a works contract. But as you said that you are merely paying him for his labour services, then from where did he get the paint. If the paint is provided by you, then its a pure labour job, but if not then both material and labour services are being transferred to you.