Since your budget is so tight skip the retro headunit for now, buy a $25 Bluetooth receiver and most of the current iPods can send FM radio over Bluetooth.

Forget the Dayton DSP. It has bad reviews.

Get the MiniDSP 6x8 instead for $275. It has free tuning software that works with the optional rta mic. Great company, people on here use them. The 6x8 model is basically the same specs and features at the JL TWK88. Connect your Bluetooth receiver to the DSP with a digital optical cable.

Your DSP will have a built in DAC you don’t buy a separate one you buy a better DSP instead.

Coaxials will be less to install but in a classic car you will almost certainly have to put them low in the kicks. By going with components you’ll greatly increase you imaging because you’ll be able to aim the tweeters at your ears where the sound goes in...duh

As for the amp wiring you’ll have a processor and 4 channels for the front so you should be running active, no bridging. The 3ch amp would be better if you're dead set on the coaxials and retro head unit, but in this scenario you really don't have a substantial requirement for the processor at all. With the 4ch in this scenario, I think of bridging as a useful option when necessary but never ideal. Pushing all 4 channels of an inexpensive amp at the lowest impedance (2ohms) will almost certainly shorten its life.

Again, the 4ch amp and the processor is the ideal solution because you can use each channel for each of the 4 front drivers.

You’re also leaving out some important and costly items from your budget. Speaker wire, sound deadener, fused power for the DSP, ring terminals, heat shrink, unique speciality tools...there’s a lot of residual things that add up fast.