I went the conversion route buying a new Losi 8T RR as a donor mainly because I already had two nitro Losi 8T's so any left over parts simply became spares for these. At the time I started the project Losi hadn't released their 8E. Also I wanted to retain the mechanical brakes & clutch which the Tekno conversion allowed me to do.
I may be treading on people's toe's here but Quattro83, over on the MaxBashing forum, has a vast knowledge of brushless conversions and gave me no end of help with mine. As a consequence I went with the MGM CompPro esc as Castle were having no end of reliability problems with their esc, although the latest V3 appears to be more reliable. Down side of the MGM is cost but the upside, if you race outdoors, is that it's waterproof which the others aren't to the best of my knowledge.
I've never tried a Tekin esc but I've only read good things about them so I may try the Tekin or Castle if I do another conversion.
I'm running single 14.8v Thunder Power 5000mah 4S lipo's with a Neu 1515-1Y motor, also recommended by Quattro83. Only time will tell how good the chinese copies of the Neu motor are that Castle are supplying with their combo deal.
If I was starting again I'd probably go for a roller and add my own electrics, not sure about the Losi stuff but it's like buying an RTR ntiro, you know what's supplied in the box is made to a budget.
Which ever route you take you it should match a ntiro on speed so decide how you want to do it and then enjoy it. It's spooky to start off with as you look at something that your used to hearing a noise form and there's nothing, just a whine as it comes past you, but don't be fooled it'll be quick!