Help me choose next purchase

Hi,

Very new player here. I’ve just kind of randomly been purchasing and playing with units so far and am wondering what unit I should buy next. I’m thinking a venom but need some advice on a purchase path to go from 500 to 1000 pts.

I currently have Mandrakes x10 Kabalite warriors 1 Archon 1 Raider 1 Chronos

I’m playing with a friend who plays tau and another who plays space wolves and we are very slowly building up our armies, maybe increasing 100pts or so every 2 months.

I’m also open to detachment advice, trying to figure it all out.

Thanks.