One tool to consider would be a track saw. They are kind of pricey, but if you think you'll need to cut down slabs or break down sheet goods, these are pretty slick. I got one to build the cabinets I posted above and it made cutting straight/accurate on 4x8 plywood a breeze. Note the price below doesn't include the cost of the track. You're probably looking at $500-$600 all-in for a track saw plus track. Alternatively, Kreg does make a "track jig" that you can attached to a circular saw. Never worked with it so can't comment on how good/bad it is.
If this isn't the kind of cut you'll need to do on a regular basis, a nice straight edge clamped to the stock like you said will do the trick with a circular saw. There are even some nice videos of people doing DIY tracks for their circular saw on YT. The nice thing about the tracks or that Kreg jig, is they control play in both directions. If you use a straight edge on one side, you can wander off the line and ruin the cut. Of course you could just use two straight edges for a quick "guide" as well.
Dewalt Track Saw
Kreg Accu-Cut
Dowels vs. biscuits: The biggest misconception with biscuits is that they add strength. They do little for strength, they are purely for alignment. That being said, they can make glue-up a breeze when done properly. Dowels provide the same alignment, but also add some strength. An alternative here would be something like the MLCS glue joint system linked below. I used that for a bench seat. While it takes some time to dial it in, it makes glue up really easy once you get it set.
MLCS Glue Joint
Finally, if you're going with a walnut table that means you're spending a nice penny on wood. So, take the time to do some research on building a table top. If you just glue it all tight together, the wood may split over the years. There are steps to follow for table tops to allow the wood to expand/contract with the seasons without splitting.