Another reason why I prefer to dimension my own timber is because I can be aware of the mositure content and condition of the timber before machining it. If there's too much moisture when you plane it up and then, you bring it indoors as a finished piece of furniture, there's a very good chance the timber will shrink, cup and distort - and possibly split - as it dries out.
Unless you're very good at planing timber by hand, I do not really think it's possible to have a complete workshop without a planer-thicknesser. You do need both, or a two-in-one machine. A thicknesser may flatten a face but, you won't get a perfect 90º edge and, unless you're working very short length, they won't come out the other end straight either. That's what the planer (aka. jointer or surfacer) is for. :wink:
Where do you get your timber, just out of interest? If you're talking about Interesting Timbers, I think they generally air dry their stuff down to 18% and then, depending on demand, may kiln-dry the rest of the way down to 12%. You should also be aware that their timber is stored in a very large shed; this won't neccessarily match the conditions of where you intend to store your finished piece of work. The other thing is that I've only ever seen three guys working there and, can only imagine it could sometime take longer than other, larger set-ups may be able to offer.
I'm not trying to knock David Simmonds and co. in any way. I've been there a couple of times myself and have had excellent service and assistance from them, paying some very competitive prices for their hardwoods too.
So, yes, there are many reasons why I believe you DO need a planer-thicknesser - and, from what we've seen of your workshop so far, it looks like you have practically everything else! :wink:
Some people go for seperate units as you can store a thicknesser under a bench, say. The disadvantage of buying a stand-alone planer though - unless you're prepared to pay A LOT of money - is that you're limited to a 6" maximum planing width. This may be fine for most people but, the advantage of owning a 10" wide planer and thicknesser is that while you can thickness 10" wide boards, you can also surface planer 10" widths as well!
If you were looking to buy new, I'd happily recommend the Axminster AW106PT from personal experience. You want to be spending at least for a decent P/T - some people will happily spend a lot more...