Welcome to the forum and Volvo addiction buddy :P Rare to see another T5 AWD owner around hehe. I have a 2005 V50 T5 AWD aswell, but Automatic. Had it for abit over a year now, and im in love with it.
For spark plugs, I just recently replaced mine. Got Bosch 0 242 236 528, 5 units. They were already gapped from Bosch factory to Volvo spec. I did re check the gap with feeler gauge and they were all spot on. The ones i pulled out were Bosch units aswell, but platinum, not the original Iridium units that comes with the car from Factory. After installing the new bosch plugs, my car ran like new basically. No rought idle or anything. Drives like new. No missfire what so ever. I got all 5 plugs for 44€ including 24h shipping which was 4.99€. Some people say only go for OEM plugs, but these ones from Bosch performs just as good. And ive driven another T5 with Genuine plugs and there is 0 difference in performance and/or noise. So if you want to spend extra in buying Genuine, no problem. But they will perform as good as Bosch units, and will last as long as they are also Itidium like Genuine. They are badically genuine but with Bosch logo
and it will save you money aswell.
Congrats on your new car, hope you will love it as much as i do :P