Just a personal opinion but if it was me I'd go all in. If as you say you have install media that already has the service pack 1 integrated into it, that's what I would use. I don't see any upside to performing a downlevel install and then downloading update after update to bring the install current only then to install the service pack, which is basically a roll up of all that you've done before, doesn't seem time nor bandwidth well spent to me.
Just make sure you don't allow Windows Update to install any of your hardware drivers. Take the time to visit your motherboard manufacturer's website as well as the manufacturer's website for any add on cards (video, audio, etc.) and get the latest from those sources and put them on a thumb drive or other external media so you're ready to bring all the hardware components up to date as well. I would even suggest that at the point when you are asked during the install to turn on Windows Update, that you opt out (can't remember the actual wording something like "ask me later" or "not at this time") until you're satisfied that you've got everything just the way you want it before turning windows update on and even then I would still configure it to never download nor install hardware updates.
Keep us posted as to how the new install is going.