I have never experienced any issues with using engine flush, but I alway use the one that you drain down the oil, fill half the contents (2.5 - 3.0 litre) and run the engine at fast idle (approx. 1200 rpm for around 5 minutes and drain down. Used this process/flush oil on a commercial basis in my garage business for years with no issues and it definitely cleans out partially blocked oilways and cures rattling hydraulic lifter problems. The 'add to oil' type flush in my mind doesn't work as well, but I cannot see how any engine oil flush or detergent flush would have any detrimental effect on an engine. The only time I would not advise to use any thin oil or flush is when a diesel engine is heavily worn and it can potentially draw the fluid past the rings and combust it, leading to a 'runaway engine'.
Engine oil quality is simple....you get what you pay for! Probably the best oils I have used are Mobil1 (not cheap) and Shell Helix Ultra (cheaper).
Spark Plugs....either NGK or even Champion (who have now got their act sorted and producing good plugs again). Would avoid Bosch at all costs.