Hey there
150 Euro will get you something really nice in terms of a place to stay, the Julis hotel is a great place to stay at but even better would be to get yourself an apartment, as then you dont have to worry about bringing girls back to your place.
Yes it is relatively safe, but its the same as anywhere else you go, whenever you leave your city and go to a foreign one obviously you need to exercise caution, viollent crimes against foreigners in Prague is rare, the only 2 things you need to be careful of is Pickpockets and Shady cab drivers, use only the cab drivers that are called do not pick them off the street.
Make sure to only carry with you the cash you will need to do what you want, leave the rest safely at the hotel, you wont get mugged but the pickpockets are good.
Clubing in Prague can be good, but of your looking to hook up with local girls then i suggest you make freinds with some Czech people before you go, as meeting the average non pro girl aint easy.
Clubs are in alot of ways like brothels anyway, as alot of the off duty pro girls go there to unwind and possibly pick up a foreigner for some paid fun, as they then dont have to give any of the cash to the club that they would normally work in.
As for the clothes shopping i have no idea, will have to wait and see if one of the ther guys post a reply for that.
hope this helped
Cheers