What SWR meter should I buy
The first question you might ask yourself is whether or not you need an SWR meter. Most modern radios, including your FT-450D, have a built-in SWR meter. I currently do not use an SWR meter for HF.
What might be useful is a combination power/SWR meter. A couple of relatively inexpensive products in this category are the Diamond SX-200 and the Daiwa CN-501H. They are both just under $100.
These products are not really very accurate when it comes to measuring power. At best, their accuracy is ±10%. They are much more accurate when measuring SWR because SWR is a relative, not an absolute measurement.
To get better accuracy, you have to move up to something like the Elecraft W2. A factory-assembled version is $300. A no-solder kit version (basically, you’re just doing the final assembly) is $250. Another, more expensive option is the Telepost LP-100A. It costs $475.