Originally posted by froque
You can get shimano Sora 7 STI, and to fix the shifting problems you will have with you current freewheel, you could find a shimano SIS 7 speeds freewheel and of course a new chain, if you search hard you can get all that for not much money.
Make sure the freewheel is similar (in tooth count) or almost similar to that one you have (To avoid problems, because the capacity of your derraileur is not known to me) .
For example if yours is 14-23 do not buy 14-29 or something like that.
Other option to consider is to go all the way to 9 speeds, but that is more expensive and your frame will need to be re-spaced to hold 130 mm hubs.
br
froque