upgrade for less than $1k

I have a sweet vintage Pioneer PL-630 which is well built, has good isolation and a decent arm I imagine since it was their higher end model at the time. Can I do better with a modern TT under a grand? I had a rega P2 a while back and was dismayed at the wobbly mdf platter, plastic sub-platter and a thin particle board for a plinth. I know the RB-250 tonearm is a good one but it didn't have a chance to shine being mounted on that TT! So, is an upgrade possible for the money? I see that keeping it under $1K doesn't even get you proper speed control. How about that pro-ject xpression III with a speed box maybe?
Rauliruegas is correct. re wire the arm, then get a better phono stage, or modify the current model you are using.

Good sound is within reach.
Dear Rotarius: IMHO the PL-630 is a good TT/tonearm item ( I owned and my unit now belongs to my brother's system. ) that maybe you can improve it with an internal tonearm re-wiring all the way down to the phono stage and of course tests different headshell wires.
The PL-630 is a trusty item.

Regards and enjoy the music,
I have a few carts lying around but use a denon dl110 most of the time, particularly for classical. Phono stage is a monolithic ps1.
Ths364, I thought Rega sound was decent, I do think my current tt/cart combo is more refined though.
I am unfamiliar with your turntable, but it appears that you like it. I am familiar with the Pro-Ject you mentioned. I had Xpression II with the speed box. Xpression has a good arm, so does Rega. It was okay to begin with but I was never satisfied with it and upgraded soon enough to Nottingham Spacedeck/Spacearm used, that cost me $1400. So I would guess if you want to spend $1500 on your upgrade than perhaps it would make sense, but under $1000 probably not. Most likely upgrading your cartridge would be the most logical. What do you use now, cartridge and phono stage?
Good question Rotarius. I have read on the AudioKarma forum pages in regards to a JVC QL-A7 turntable that I have that I could spend over a thousand dollars on a new tt and be disappointed that it was a not of equal caliber to the JVC.
If you're gonna spend two-hundred on a cartridge that leaves eight-hundred for the tt.
You didn't mention whether the sound quality of the Rega was
to your liking-just that the build quality was disappointing.
I think that a lot of people are rebuilding Lenco, Thorens, Garrard etc. because they've had a similar experience to yours.