I am not sure there are any freehubs with 126-mm spacing. I haven't seen any. So in a 126-mm aluminum frame, the poster is pretty much stuck with a freewheel and seven cogs max. On the steel frames, i'd stretch them and go for as many gears as possible withing budget.

Search online, watch the Returned Products and Closeouts pages of bike sites. You can find new 9-speed Tiagra stuff at a pretty reasonable price---except the brifters. I got a couple sets very slightly used after very careful shopping on Ebay for about $70 including shipping,. I just had to be really patient and lose a lot of auctions after I hit my own top bid.

Going to 10- or -11-speed means buying new pretty much and the cost is a Lot higher.

Oh, and probably you can replace the two bad cogs on the Tiagra 9-speed cassette pretty easily too.