NEX5 + LA-EA1 + SAL75300 = no AF
SEL18200 maybe more expensive, but then the AF is "SSM-like" (better than SAM, according to reviewers) ... so you can't compare it with SAL75300 with in-body AF. There are also some other differences as well, not just the AF.
Personally, I'd rather wait for the rumored SEL55200 ... at least it should be similar to the kit lens SEL1855, or hopefully better IQ/performance.
For reference, copied from Sony:
http://support.sony-europe.com/dime/downloads/downloads.aspx?f=FW_LAEA1_V02_WIN&site=odw_en_GB
Support for the autofocus function with A-mount lenses
The following SAM and SSM lenses are supported*:
SAM lenses
DT18-55mm F3.5-5.6 SAM [SAL1855]
28-75mm F2.8 SAM [SAL2875]
DT55-200mm F4-5.6 SAM [SAL55200-2]
DT30mm F2.8 Macro SAM [SAL30M28]
DT35mm F1.8 SAM [SAL35F18]
DT50mm F1.8 SAM [SAL50F18]
85mm F2.8 SAM [SAL85F28]
SSM lenses
Vario-Sonnar T* 16-35mm F2.8 ZA SSM [SAL1635Z]
Vario-Sonnar T* 24-70mm F2.8 ZA SSM [SAL2470Z]
Distagon T* 24mm F2 ZA SSM [SAL24F20G]
70-200mm F2.8 G [SAL70200G]
70-300mm F4.5-5.6 G SSM [SAL70200G]
70-400mm F4-5.6 G SSM [SAL70400G]
300mm F2.8 G [SAL300F28G]
* With an A-mount lens attached, the autofocus speed will be slower than when an E-mount lens is attached. (It takes approximately 2 to 7 seconds). It may vary depending on the subjects or brightness of the shooting environement.