Hello Dinah,
first short comments to lenses you have alread:
- SMC Pentax F Zoom 3.5-4.5 35-70 mm
This is a good lens, small, light. The macro-function isn't really macro but close-up.
- SMC Pentax DA 3.5-5.6 18-55 mm AL
This was the Kit-lens you got with your K10D. It is really outdated. The worst of all DSLR kitlenses ever!
The later 18-55II or the WR version where better.
- SMC Pentax DA 4-5.6 50-200 mm ED WR
Some had better experiences with it, I cannot really comment on it, never tried it for long enough.
- Pentax Power Zoom FA 3.5-4.7 20-80mm Came with the Z20
Must be 28-80 and not 20-80!
Heavy and the worst of all of them. But you can use it as a doorstopper or paperweight
So the lenses I recommend:
Cheaper but very good for the money:
Zooms:
- Sigma 17-70 2,8 (HSM not really necessary, makes it quiet but heavier) (There is a Tamron 17-50/2,8 supposed to be good but I never tried it)
- SMC Pentax DA16-45/4 (one of my favourite zooms, on the K70 and KP it shows what it can do, one of the few zooms I use myself)
- SMC F-70-210/4-5.6 (better than the Kit 50-200 as I know from a friend)
Primes:
- SMC Pentax DA35/2,4
- SMC Pentax DA50/1,8 (I prefer the SMC F50/1,7)
pricier but ... worth it:
Zooms:
- SMC Pentax DA20-40/2,8 limited (one of the three zooms I use) limited zoom range but really great quality.
Well, you have almost a DA21/DA35/DA40 in one lens!
- SMC Pentax DA55-300 ED PLM (the other of the three zooms I use): super fast and light enough
primes:
- FA43limited (quite a beast, needs some attention as it can be a bitch... but a very special one)
(I would add the DA*55/1,4 SDM but it often, and don't listen to any others advice who claim it doesn't fail,... it often has failure of the SDM motor and then it is an expensive but wonderful non-AF lens!)
- DA15 limited (nice light superwideangle) (DA14/2,8 is great as well, as good as the DA15ltd.)
- DA35/2,8 macro (one of my most used lenses, great 35mm standard angle, superb macro but a bit slow AF because it is a macro and thus:
every macro has a long focus-throw.
- D-FA100/2,8 macro WR (now cheaper as a new version shows up)
In a way with just a DA16-45/4 and a DA55-300 PLM you can do a lot!