I use the one olds linked to, the Radio Shack one. They show the price at $49, but if you go to the store they are usually $39, and they often run them on sale for $29.
I wouldn't recomend an on-board because if you have more than 1 car or r/c product it is a pain to keep switching. I use the duratrax hand held lazer pointer.
On boards can be a pain in the %$#. They easily get dirty with oil and grime. It will coat the temp reading wire and throw your readings way off. I would definitely get a handheld IR.
I use the duratrax flashpoint,it's a great temp reader at an awesome price.the ofna version looks really good too and it has a distance to spot ratio of 3:1,as compared to the fashpoint's 1:1 ratio.this means that the ofna will take a reading off a smaller area at the same distance,this is good for getting a more accurate reading off the glow plug and not the area surrounding the plug.
It will fit but trust me an onboard is going to get dirty and give you bad readings. Even up to a 60' difference. A friend has an onboard and he is constantly having to borrow my IR b/c his temps are way off. They get dirty easily and the temp wire can't get a good reading. For $25, the IR is the cheapest and hassle-free way to go.
Here is a link to even save you $2 since money seems to be a factor for you.
Duratrax IR Temp Guage for $23 instead of $25.